MA strategy infodump since that's how i went thru bitterblack and ran into similar 'wtf i'm not doing any damage now?' walls
are you using ricochet arrow? it’s not the most useful skill in the rest of the game but in bitterblack’s confined hallways it really really shines. each bolt can bounce like 15-20 times, gains damage with each bounce, and the upgraded version fires 3 bolts per cast. the big tanky “miniboss” monsters you run into from time to time are usually vulnerable to and/or stunnable with lightning. there are even some open arena-ish encounters where you can bait dangerous mobs into chasing you into a hallway then shred them with ricochet spam
similarly, actually fighting with daggers as an MA is only situationally useful but they give you a few great buffs. magic rebalancer is absurdly good skill in BBI: it applies a (stacking!) magic attack + defense buff to you and any pawns that step into its area of effect and any undead mobs get their magic defense debuffed by it (less useful than the buff but really helpful in those rooms full of tanky zombies and certain bosses.) the attack buff applies to all your bow skills, immolation damage, staff spells, certain dagger skills, and will boost magic damage from any enchanted weapons. its duration isn’t long enough to keep up all the time but it can help a lot in tougher encounters. also, even though your staff magic is pretty basic, a buffed high comestion can hit fire-weak enemies really hard and can stunlock some monsters, so sometimes it pays off to equip a staff.
speaking of undead! magical gleam is another MA bow skill that’s underrated in the normal game but really useful in BBI: it damages and stuns undead in its radius (including doing sneak attack damage to those zombies laying on the floor you see sometimes) but it’s most invaluable for keeping those rooms full of standing water illuminated without having to stop and re-light your lantern all the time.
gear wise you basically want to equip as much magic damage armor + rings as you can, while keeping an eye out for good daggers. your bow is pure magic and physical dagger damage really only matters for normal attacks and thousand kisses. full physical daggers could wind up doing more damage for you with the right enchant from a pawn tho, so ymmv.
you will eventually start finding lv2 BBI gear (the ones you have to identify outside the dungeon) that are wildly better than anything you can find outside (lv3 stuff even more so, but it doesn’t start showing up til the lower strata iirc.) you’ll also start finding rings that can unlock a third tier for your skills, so keep an eye out for them, they can make a big difference on skills like your homing shot (adding more seekers) and ricochet arrow.
there’s also a pretty decent jump in difficulty between each strata (the game doesn’t call these out specifically, but basically assume each time you unlock a shortcut from the main area, you’re in a new strata) so you’re probably running right into one after beating the gazer. each dip into bitterblack should get easier as you pick up gear that wasn’t available in the previous strata.
also, one last tip sorta related to the last one: sometimes you just gotta run away! one of the reasons i like bitterblack so much is how it loves throwing you into situations that any one build or party setup can’t fully conquer, especially without prior knowledge. it’s tempting to grit it out because the rest of the game doesn’t really put you in those situations, but getting the hell out and coming back later when you’ve had a chance to prep (rework skills, hire new pawns) is often the best idea. there’s also a lot more optional paths than may be apparent at first, so if you’re getting fucked up it might be a sign to back off and try to find a path better suited to your current party and gear situation. you can often stumble on gear upgrades this way, there’s tons of cursed gear chests hidden in diversionary paths off the main route