Funny you should add a quest for Assassins in Creeplight when that's just what I've been working on. Even after today's changes my strategy still works:
It's not displayed in the results, but I believe I killed Anoobis' first form at level 3 or so, with poison + fireballs, then used Mana Leak and first strike to level up while continuing to fireball him to death. Note that this was with the boss at edge of the screen, meaning no first strike against him (that's a bug that will be fixed, right? I don't like that at all) and
using Mana Leak made Mystera angry and gave everything extra magic resist for some unknown reason. Plus, this was with a mere 60 piety Mana Leak, and I didn't have BLOODTOPOWA or IMAWALL (which are two of the most helpful glyphs in this strategy). I don't think Mana Leak necessarily needs a huge nerf, especially since classes other than Assassin and possibly Rogue will have a harder time exploiting it and Creeplight is fairly unique in having no enemies with first strike or physical resistance which would both pose difficulties, but decreasing the ratio of damage done to 3/4 piety or maybe even 1/2 piety would probably be a good idea.