You get like 2% DMG-bonus for 2 seconds with 2 points in Blast Cannon. It doesn't seem good enough for me. It's either Blast Cannon, or Fussilade, but yes, Fussilade is lvl 35-or-what, and blasting away even the Act1 spawning bosses is a chore, so picking up Blast Cannon for 1 point is more optimal in my book, even if you plan on Fussilade.

Also do not forget, you are limited by clvl 55 and Eminent karma for the first playthrough if you do not grind. So any point into Blast Cannon (when you aim for Fussilade) will have to come off from somewhere.
Immobilisation Copter maybe? You loose the rank, but that nr msinly just slows things down?
And how well Blast Cannon synergises with spider mines?

EDIT: it's a bit of a letdown you can't find solo guides for T2, only lvl100 multiplayer character setups.

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Before I forget, just a minor trick I got struck with. There is that dummy in the hub-towns. And I had that unfortunate happenstance when entering that dungeon. As town portal scrolls are not hard to come by, and aren't overloy pricy after the very start, AND I don't rly use them all that much, especialy with Engineer, who has its own healing devices, why not set up a tp before entering a dungeon, max out your charges, cast Force Field, and go in that way. Just a bit of extra insurance.
Also for Veteran, as soon as you can go to the enchanter within the town, just upp any leftover potions you have to the current shop-offer, or something, definitely step up from basic potions, or rank 2 potions after Act 2 etc. For that extra bit of kick.

Now entering and cleaning the arrival-vicinity is NOT reliable, the Dwarf Mine simply respawned when I tried that. Bah!

Spider Mines are very-very suggested for any hardcore Engineer, as you can release them (and they WILL rush in) from "out of sight". Fighting Chillhoof, that act 1 grizzly boss with its buffed Northern Rage variant spell becomes almost trivial as you can just run around throwing drones. I still find it curious why none of the robots have HP to tank for you. Althogh the spiders do have HP? They definitely have when the enemy is using the skill.

Here's a page from a T2 wiki on Blast Cannon https://torchlight.fandom.com/wiki/Blast_Cannon and one for Fusillade https://torchlight.fandom.com/wiki/Fusillade so that you can compare them for yourself. From my POV, putting 2 or 3 points into Blast Cannon can really pay off, until you reach level 35 and can access Fusillade, but of course, YMMV.

As you can see from the charts, you get 4% DPS for Blast Cannon with every point, on the repeated shots. That's not very exciting, when you can get Spider Mines instead.

Also highly important WHEN you would invest more into BD, as you want etra protection ASAP from Aegis Shield along Force Field because of the entrance-problem.
Healing Bot is also necessary for the HP and equaly importantly mana regeneration.

I just don't see where you'd insert more Blast Cannon points.
I just finished Act 1 at lvl 21, Well-Known, and have 1 point put aside. 1 point in the charge-skills, 1 coup, 7 Helaing Bots, 1 Blast Cannon, 5 Spider Mine, 1 Gun Bot, 4 Aegis Shield, and I'll need as many point in Aegis as possible before entering dungeons (9 point would mean 10% activation, what is a gamble, but sure improves survivability). Force Field is also prioritised.
EDIT: lvl 26, and 0 unspent skillpoint.

See? I'm not against the investment, I just need more info when to do it, and what to sacrifise from the preconceptualised skill-build to still remain effective. Efficiency is my sole priority.

You write both that you have one point set aside, and that you have 0 unspent points. Also, you and I have different goals in mind. Being hardcore, survivability is your primary interest; as I never play that way, I'm more interested in a well-balanced character that can win fights and (usually) survive long enough to do so. I'm not trying to force you to use Blast Cannon, just appreciate that it can be more useful than you currently think. And, consider this: if it can demolish a small hoard of monsters in one shot, they won't be able to do you any damage.

Read it again: the leftover skillpoints are on different levels. I had 1 at lvl 21, and 0 at lvl 26.

Seems you don't understand the concept of hardcore, or why I am going that defensive. Shame.

On Blast Cannon: no, it can't. Not on Veteran+. You usualy need 2-5 shots, even if you pump STR exclusively (which oyu probably shouldn't, as half the unique cannons have VIT-requirement too). And anyway, the skill works equaly well at 1 point than at 2-3 points. Heck, as told above, you don't even gain any advantage if you can destroy those hordes in 1 shot, as Blast Cannon at more skillpoints only give your extra damage if the enemy is already hit!
Not to mention when you have Spider Mines, those hordes rarely turn thick enough to require Blast Cannon. Smetimes they do, so I learnt to use the TAB, but it's not regular.

Oh, no, I understand exactly why you've gone heavily into defense: you only have have one life. What you either don't understand or don't care about is the fact that there are many places in Act II where you need to clear out a mass of bodyguards and Blast Cannon with only 1 point can take too long to do the job. I'm not suggesting that you max it out, especially if you're planning on using Fusillade later but putting up to three points in it can help you survive until that time.

I forgot about Act 2's Bone Prison. Those are waaay too massive for their own good, and if one is in a boss-arena, you can't even tp out of it :(

So back to Act 1.
Zombies hit way harder then anything else, so don't let them sorround you.
Also, it IS possible to get through Act 1 without ranged weapon, if you are very careful. Wouldn't try that in Act 2 though. Champion/boss monsters are a problem, but you can run around like idiot while the healbot patches you up for another strike. Still, probably better if you have a ranged weapon on switch.
Melee weapons are very likely outdamage any canon you can lay your hand on, so at least until you get to the last open area of act1, try going with those. It's sufficiently safe. Then come the spawners, and they are a nuisance, so Blast Cannon can help you out there.

For spells - unless it is lowering restrictions, or give other passive bonuses, you are likely better giving them to the pet. Bargaining and Animal Hadling also should go to the pet.

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Icedeep Cavern (I'm not even sure which act the cold area is, sry) should be taken last, at lvl 20-21, with 1,500+ HP and 75 ice resist on elite. Otherwise it is bloody dangerous. Spider Mines thrown mindlessly while running around like headless chicken wins the day. You should use up some gems to buff up your resistence. And grind a level or two if you feel being damaged too much while on the first floor. The dungeon respawns every time you enter.
It sucks that melee weapons come with double the damage cannons get.
On skills: Healing Bots are a must, Spider Mines ARE a must, 1 Blast Cannon is a must. Still think the 3 top passive worth the point. Force Field will be required in Act 2 to survive entering dungeon levels. But Aegis shield did not work out for me, it'd depend on Armor, but my priority is STR, because cannons are like the only source of damage here that are not suicidal besides the robots. Not sure where to investt he points. Blast cannon maybe, or Bulwark, or Heavy Lifting, or Fire and Spark.Storm Burst would solve the Bone Prison problem, but comes too late...
Hm. If I would drop that, that... How about 7 Healing Bot, 5 Blast Cannon, 5 Spider Mine, 2 Gun Bot, 5 Force Field, 1 Coup de Grace, 1 Charge Domination for the ... 1st Guardian fight? (Act 1 end)
For that fight you need 1,800+ HP, clvl 22, and 20+ 2nd tier potion each. Huh, this was close.

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For Veteran, Spider Mines solve pretty much everything, occasionaly thrown in a Gun Bot against bosses/champions. Elite is more problematic, in Act 2 not just the blocks, but immense amount of HP will stand in your way (Bone golems and champions are a year to knock down). This somewhat seems to be possible to mitigate with different damage element, but as there are no charts what hits what, that remains a guessgame.
Btw, with the berserker I've noticed Act 3 deals a ton of poison damage? Will have to watch for this, as some of the things don't reveal themselves, as the game though hits harder the later stages, is also vecoming more simpler? I dunno.
Anyway, and melee build until proven otherwise seems entirely impossible on the higher difficulties, because of how much damage the enemy deals with every strike.

For the cannon-engineer also most of the skills are simply unavailable because of item-requirement.
Eg. from the Blitz-tree you can have Seismic Slam, which was a pretty low range. Ember Reach simply pulls thing in melee range, what is a terrible idea. Storm Burst can be an excellent escape-skill, but it come sooo late.
Even from Blitz's passives you only can pick 2: Heavy Lifting, which is fine, but I think eventualy will become obsolate, and Coup de Grace, which is fine, but the chance the enemy will get stunned is not THAT high.
So the Blitz tee does offer a couple of things, but what it does offer is very mid.

The Aegis skilltree also restricts access for being the shield-user tree, but cannons are 2-handed.
There is of course Force Field, which is very good on lower diffs, and serves as emergency-skill on higher ones.
Overload is an option, but relies on charges, and has a short range (we had a similar skill on Blitz).
Dynamo Field is another melee-range skill. I mean these can be useful if you get stuck ina Bone Prison, but I hope we don't need massive skillpoints just for that...
Tremor is example of very badly describing what a skill does.
And Immobilization Copter only slows enemies, what is good, but you know.
Passives are again 2 out of 3, and Aegis of fate is simply a gamble, plus would require intense point-dumping into Vitality,. what would be nice, but then how I kill enemy? And while Charge Reconstruciont does not sound useless, now I see it as doesn't do enough.

The Tar Phase Beast challenge is though. Again: Blast Cannon needed (to get rid of the spawns), plus Helaing Bot (to maintain mana), plus a load of Spider Mine (when your chosen boss is not covered by spawns run to it and throw at it).
Previously I thought you must just step on the smallest tars, else they re-merge the bosses, but nah, it's worse. King Tar seems most vulnerable, then Mister Tar, than Doc Tar.

Let's go through the last skilltrree: Construction.
Healing Bot is a boon, and you should prioritise it over everything else.
Blast Cannon is still not my favourite, but it has advantages, especialy on Elite, where Spider Mines don't make it obsolete at all. It eliminates crowds faster than otherwise, plus it costs minimal mana, so I'm heavily reconsidering this.
Spider Mines are awesome, too bad they are not enough on Elite. On Veteran though you can get away with "just" them (Gun Bot support still required against bosses/champions).
Shock Grenade is the worst thing, don't waste your points here.
Fussilade... Comes late, and each shot costs as much as a drop of Spider Mines. It only gets good with at least one rank under it. The main advantage is the endgame damage and the fire-and-forget aspect (homing missiles). It is "1 target" skill, but is still a blast AoE. I dunno, it feels you should have it, but on the other hand only during NGs? It rly comes too late all things considered.
Sledgebot is another addition to your army. Again the description does not convey the necessary info clear, but found a video, so tier 3 bot won't just become ranged attacker, that's a relief.
All 3 passives are open on this tree: Bulwark is fine I assume IF you somehow manage to do an all-vitality build, but I just can't imagine that on Elite, or even Veteran, so do what you want. Fire and Spark seems meh, especialy that electric damage is rare (tier 3 copter do a bit), but the fire-damage allegedly applies to the spider Bots, what can make it a worthy investment.
Charge Domination barely does anything when dumping more points into it, but for 1 point the chance to go from zero to hero in a blink I definitely think that gamble worth the 1 point here.

So the new plan is:
- 1 Coup the Grace
- 1 Charge Domination
- 10 Healing Bot
- 5 Blast Cannon
- 10 Spider Mines
- 10 Gun Bot
- 5 Sledgebot
- 10 Force Field
- 5-10 Fire and Spark (debated wether this buffs the mines or not!)
- 0-5 Immobilisation Copter

Huh. I met a champion which walked toward me, then just disappeared into the hill under us.

Also, seems if you lead a monster to a corner, the AI just might get stuck there, walking in itty-bitty circles and not attacking.

Fire and Spark according to the ingame skill-sheet has no effect on Spider Mines unfortunately.

So the new plan is:
- 1 Coup the Grace
- 1 Charge Domination
- 10 Healing Bot
- 5 Blast Cannon
- 10 Spider Mines
- 10 Gun Bot
- 5 Sledgebot
- 10 Force Field
- 5 Immobilisation Copter
- 15 points to freely allocate. You can rise some of the above skills, or dump points into Heavy Lifting (I wonder if it only works with the basic attack, or all attack requiring cannons), it might even worth to buff further Blast Cannon, or even start Fussilade. Also, Storm Burst probably worth a point.

There is a trap that has no animation, and does damage straight on your character. What a bummer.

I think "totem"s (Act 1 ~buildings, only voulnerable to physical damage) only exist on Elite.

Stepping on a rat-or-something also counts as kill, so you can get charges from those.

Start to hate the snwoland. Blocking, charging, massive elemental damages, hidden enemies...
At lvl 17 spend your skillpoints to Hwaling Bot, Blast Cannon, Spider Mines, Force Field, and 1-1 on the two passives. That works. Though you'll need an upgrade from 50 dmg cannons when you fight that boss... its namestarts with an M, one of the green lands. And preferably another upgrade when you enter the snowland to 80+ (and your ice resist to 45+ by gems or something). Also around this time Heal Bot starts to heal frequently enough thatt he pet will always remain in battle, so setting it to offensive, what is in demand for elite anyway becomes really contributing to your success.
Also, for the last two dungeons if you HP does not surpass 1,400 when you enter either of the two, go to the shop and buy better gear.

Advice: if you don't know what1s happenning (too many things on screen) - drink a health potion.

Note: there is a bit clipping issue what you can abuse, especialy if King Kogg (or what, in the dwarf mine) comes to the edge of its platform. Though he's 2 story higher, your mines can hurt it, while it can do nothing to you.

A thing I found out is, why no "play solo on Elite" guides exist is, because people make their character on whatever difficulty they want to brag about, then create "multiplayer" game on easy, grind up, and done. That explains a lot.

Huh. The Forsaken Vaults is a though cookie on Elite. One: I constantly forget the floor-traps and keep dying because of that. Two: the monesters are really unfair. 2,400 HP is "just enough", and I pretty early found an Adventuring-scroll, what I think pretty much overlevelled my by 1-2 levels at least!

Woah, no wonder the machine-dungeons feel though! Those drone-spawners spill lvl 10 immobilisation-copters!!!

I'm done. lvl 30, 2,350 HP, all the advantages, and the Artificier simply twohits me, even though I was drinking the largest potion possible, and everything. This just doesn't work without equipment-grinding.

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So the full-VIT Elite Engineer sucks in many aspect, primarily that its cannon will be mostly useless. But I got lucky - so to say -, and got a cannon with +MP per hit, what allows to lay mines almost without interruption. This weapon-attribute does NOT work with skills though, so Heavy Lifting is back on the table again. I also think it steals that amount of mana per enemy hit, so that's obviously funky.