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By level 25 you should have already acquired Merhune's Dagger. Do what thieves are good at. Backstab out of sneak for a critical one-hit kill.
Krosis' mask DEFINITELY helps.
But yeah, the Bloodskal Blade's projectiles deal bonus damage to Zahkriisos, 30 bonus damage if the player is less than level 50 and 50 bonus damage if the player is at or above level 50. Use what the game gives you. If it gives you a good sword, there's probably a reason you'll need it - not JUST to open the doors.
Also, enchanting your bow with FROST will help - frost drains stamina ... no stamina, no ability to cast magic fireballs of mass death. No magic, easy boss fight.
Most people say Zahkriisos is one of the three TOUGHEST priests to kill, especially because he can level WITH you up to level 60, but I found the fight an absolute JOKE. (I actually ONE-shot him.)