Admittedly there were some cases where it's just impossible to complete it (Fighter vs Bleaty without CYDSTEPP or other useful equipment), but overall it's not too bad.
If you're doing purist and
get a bad run of glyphs and
you don't get a good alter or shop, maybe that's impossible, but if you have any
half-decent assets he really just raises the difficulty of the level up to "hard", which is still quite reasonable for a veteran.
I actually just played Doubledoom with fighter and got Bleaty and Jormungandor as my bosses. I chose preparations to stack resistances (Dragon Shield + Patches), which eventually gave me enough staying power to survive a blow from Bleaty. Along with HALPMEH and a mid-battle level-up from killing other monsters, that let me beat Bleaty without even using potions or death protection. An Orc with PISORF can also slaughter Bleaty, and any non-berserker who gets his hands on a full-piercing wand can fry that goat.
Certainly Bleaty will trip up a beginner, but veterans should only lose to him if they get some additional bad luck.