Interesting strategy - been a while since I played A100 or a pistol build (or much at all), but I have fond memories of Sharpshooter builds for that challenge. Combine that with HR and INT and you can get yourself out of harms way and snipe a ton of enemies off before they become a problem. That worked on HNTR though, don't think I ever tried higher difficulty A100 runs.
Pistols can still work on higher difficulties in A100, though the early game problems get really exacerbated in A100. I've done Sharpshooter in UV A100 before, but I think a Scavenger works best for a Pistol build in A100; you can get traits to help you survive the early game, and then get SoG when you find a pistol actually worth using, while Scavenger lets you get all the rare mods that really benefit pistols (if you ever want to achieve the dream of wielding a P3N2S2 Combat Pistol or P3N1S2 Blaster, Scavenger makes that actually possible without playing A666). You lose out on Dualgunner, but I think Dualgunner is a trap anyway on higher difficulties without a Nanomachic Pistol or Nano-modded Blaster, where you'll far too often empty your pistols while several enemies are still in range, and so a single souped up pistol + ammo box is much more reliable. Cateye is another nice A100 pistol build option if you want to keep Dualgunner and want to stack SoG with SoB, while having a mastery that still helps pistols. Not too viable in the standard game as it requires so many traits (which will make you either not get Cateye or your pistols up to speed until very late), but in A100 you got more than enough time to make it work, while having a much smoother early game than with the actual Pistol masteries. I once did a UV A100 + AoMC run with Cateye where I finished as a pistol build and it was quite fun.
My most hated enemies by far were the VMR and the Nightmare Arachnotrons (which came probably the closest to ending me in the second big cave near the end). I ended up using a Hellwave Pack to even the odds against a mess of Viles and Mancs that were too tightly packed together for me to get at the Viles easily. I also ended up having to fight two Cyberdemons near the end of the run, one on an island level where he was right in the lava, and another in a regular level, and in both cases, they took two full double-mags to bring down.
If you
really hate VMR, I would strongly advise picking up Berserker in A100 runs even if you have no intention to use melee (and it's just nice insurance in general to protect you in case you do get ambushed by a big hit or accidentally push a Napalm Barrel into lava). Archvile zaps will always trigger it, and Mancubus and Revenant rockets will trigger it nearly half the time, while when it does trigger, berserk's +60% resistances are applied
before damage calculation. This is especially powerful with the 0.9.9.8 Marine, as those resistances stack on top of their inherent 20% energy resistances, and so you're basically running around with inherent 80% resistances against them
before your own armor is factored (this reduces an Archvile zap down to 4 damage, and even a max damage Revenant/Mancubus rocket won't deal more than 5, while unmodded Red Armor or even weak Nanofiber Red Armor brings that down to 1). It's not just VMR either, high damage rolls from other several hard-hitting enemies, like Barons and the bosses, as well as barrels, and your own splash damage, will trigger it too. And of course when you're berserked, you're now much faster at everything to help you kill them easier (or escape if needed). Alternatively, if you don't want to get Berserker (or before you can get it), just simple Fireproof Red Armor will neuter the VMR's threat; with a Scout/Technician, it'll reduce an Archvile zap down to 5 damage and Revenants/Mancubi won't do more than 7 at max, while with the Marine's energy resistances, it'll just outright reduce their damage down to 1 without any mods nor TaN.
As for Nightmare Arachnotrons... yeah they're the biggest bastards and there's no way to effectively eliminate their threat, while they can't trigger Berserker either, and even if you got the defenses to reduce all their hits down to 1, you still won't be able to tank a swarm of them. If you spawn into a Nightmare Arachnotron cave and have Berserker, you could facerocket yourself to at least trigger it and speed yourself up, you really need speed to not get overwhelmed by them. They're also a very big reason you want to keep Tactical Boots or other speedy boots handy and don't want to only have a pair of heavy boots (especially when you got no Hellrunner...), speed is crucial to survive them and if each movement takes you over a second, a swarm of Nightmare Arachnotrons will just gun you down as you try maneuvering to better positioning (at least on HNTR, caves will have relatively few enemies and so you can more realistically stand your ground against them, but if you ever try A100 on the harder difficulties, definitely don't leave behind those speed-boosting boots).
I entered the final level on an Invulnerability, and since it was another Pitch Black level, I decided fuck it and busted out the Angel Arm for the first and only time on the run to finish things off with a bang.
Darkness levels are very nasty, especially when you don't got Berserker to protect you if you stumble into a big hit, so I would say just nuking it was a wise decision!