The basic idea is, find a quick MA altar (the "scout altars" prep helps with this), then start casting your ass off to build piety. You start with APHEELSIK, which (IIRC) provides four piety at the cost of eight mana. So: cast it, explore a bit, cast it again... rinse and repeat until you're at 100 piety. BLUDTUPOWA helps, as does LEMMISEE (you regen the mana it takes to cast it when you cast it, so it's basically just a gain piety button). When you're at 100 piety, finish exploring the rest of the dungeon, so you see all the monsters (and so they can't regen when you explore more). Then go down to the basement, and kill the sacrificial goat - all the cultists will turn into shades. Then go back upstairs, and blow your entire wad of piety on the Mana Leak blessing. Everyone takes up to 99 damage, instantly - which is enough damage that even a level 1 assassin can one-shot through the rest of the HP a level 7 or 8 shade will have (and pretty much everything else lower than that, too).