I mean there's really only one boss, but he's just got multiple stages
The answer is that you're close, but not quite there. If your strategy isn't capable of regen fighting forms 2 and 3 then you're probably going to run into some trouble before the end. As for where you're falling short and how to fix that, here's a few scattered items on the third segment:
-You absolutely want either 3 lifesteal or 25% physical resist and 2 lifesteal. It makes an absolutely staggering difference in terms of regen fighting ability. This usually means avoiding bankers like the plague and *maybe* cleaning them as popcorn later if you can afford to without getting punished. Alternatively, Stone Sigil is a pretty great grab in part 2.
-Try to keep your permanent damage (ignoring momentum) *below* 97. This will allow you to milk your level 7 goats repeatedly throughout the fight. Furthermore, they pile up over the forms. This means that by the third form you can have 3 blood cows to repeatedly refill off throughout the fight. The third layer of lifesteal also makes this stronger.
-Speaking of, generally do everything you can to save those goats as long as you can. They offer momentum damage, potential mid-fight level up, and health refills. So save them for a form that is really giving you trouble (typically not 2 or 3).
-Other items that help with regen fighting or survivability are great. Venom dagger, for example, basically provides +20% damage against the boss for small slot during the entire fight. Platemail is obviously fantastic. Probably avoid Whurrgarbl though, as the burning can screw you over when PISORFing to break his shield in form 1.
-Keep that PISORF. Keep it and hold it tight and never convert it. You will need it at the very end unless your run is going exceptionally well. Similarly, I dearly hope you have at least 1 fortitude tonic when you descend that ladder for the final fight, if not 2. You're going to want it. A single salve can also be handy.
However, more than likely the efficiency you need to make up is in parts 1 and 2. Although part 3 is epic, has a fully unique monster set, and ends on a ridiculous boss, the play of it is pretty straightforward. So instead try to get out of part 1 with a 2nd or even 3rd glyph (for example, bringing HALPMEH+B2P/CYDSTEPP/WONAFYT/LEMISI for part 2 and then keeping the HALPMEH to part 3 is great). Clear part 2 with 3 sets of quicksilver and reflex, or some great items. Make sure your locker prep has good value through all three parts (I think I used Cursed Cutlass?). Ultimately, since it's a triple quest, small improvements in play--especially in part 1--can snowball hugely over the course of the three runs. Keep me posted and feel free to ask for more tips for other segments if you're having trouble.
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