The absolute easiest way I can think of would be running an Assassin, and scumming for the sub dungeon in the Swamp to get in there at a low level. Mischief is always your level, so the lower you are, the more you can abuse Swift Hands. Enter, kill Mischief as much as you can until you can't kill him without getting hit, go out, level up, kill Mischief, level up, rinse and repeat.
Alternatively if you don't want to cheese it to much, a Rogue should do as well, takes care of first strike at least, Flame Magnet for a fireball and you should be pretty much set, at least, that's how I got him.
I just found the biggest trick to making it easier was finding the subdungeon at a low level, so you can always just level up to get past the strong zombie Mischief.