What about sharpshooter? That's 75% more crit damage with Critical Power.
But the main issue is probably the fact that not only crossbows don't get any armor piercing ammo, they incur 125% mechanical damage resistance, so they are at a double disadvantage.
From my experience they are only good at picking off lone enemies without aggroing others (because silenced guns suck) and have the best synergy with traps, but that's it.
I have sharpshooter, too... and all the relevant crossbow feats except for Snipe (not that useful for my playstyle, and wouldn't help much here either). Crossbows actually work OK until Deep Caverns, and even there they only really fail on industrial bots, deep worms, shroom people, and Tchort. Unfortunately, most of the aforementioned enemies must be dealt with at one point...
My crossbow user is just about able to kill the tentacles faster than they regenerate the damage... in theory. In practice, unless it's just a single tentacle, she'll get killed first. Crossbows aren't exactly great as a primary weapon. Not sure how exactly a crossbow jockey is supposed to win that fight, haha. Many stacks of poison, maybe? If it even affects the eye... I don't know...
It does affect the eye, yeah.
I have a battleplan for this, it'll just need a lot of preparation and grinding. It also only really works if you're a crafter, but a crossbow user pretty much has to be one anyway.
1] Scour the ENTIRE deep caverns to get all materials that could be used to make shock and incendiary bolts as well as gas grenades and cryo grenades, put them in a big pile somewhere
2] Fish for a few hours to get materials for combat buff meds
3] Figure out a way to kill at least one deep worm (this is a blocker for now, but may be optional) for corrosive acid and gizzards required for a lot of medicine
4] Craft tons of high-end incendiary and shock bolts as well as some burrower and cave ear bolts. Gas grenades, cryo grenades and molotovs / magnesium grenades too.
5] Get enough oddities to level up and put 40 points into throwing (it's at 0 currently). I'm not sure this is possible, especially if I can't get oddities from deep worms [assuming they have some].
6] Get all the crafted stuff, do the mutagen puzzle
7] Deep breath
8] Enter, immediately stealth and dash over to left, kill mutagen tank, hide in shack, pop out to kill other mutagen tank as well, preferably with stealth + TNT. Tchort enemies aren't a big deal with traps, just keep tentacles controlled with fire and kill when appropriate.. especially the freaking Bilocation one, that one is pretty much gameover if it gets a single cast off.
9] Trap mouth of tchort with spirit poison trap (to summon bilocation clone, hopefully it'll tie up enemies and beat on them as well) or arrow
10] Kill tentacles, move to other side, kill tank, move to safe spot
11] Drop all the damage-over-time effects of the world on tchort. Gas grenade to stack vulnerability, burrower bolt for armor-ignoring damage, cave ear poison to increase all this damage even more. Only use high-grade incendiary and shock bolts on tchort, normal bolts are for mooks. When special bolts run out, retreat to resupply and repair crossbows (YES, your crossbow will break, multiple times. Proven.)
12] ... profit eventually? Assuming tchort gets 0 regen when all tanks are destroyed, that is.
edit: doing the good old Caltrop Caper (cover the arena with crawler poisoned caltrops and wear steel boots for immunity) may also help, though I suspect the tentacles are going to be immune as they appear in non-walkable tiles to begin with.
