I've beat hell on this once. Back in one of my high school classes where every day you had like 15 minutes of work in an hour and a half class, so everyone just played games or browsed sites or stuff.
I'm stuck on the Undead Core. Got the Spur and Booster 2.0, Timer, best ending ready. Waiting to beat ALL THREE OF THAT DAMN DOCTOR'S FORMS so I can get to Sacred Grounds.
I just take it slow. Take out the transformed Mimiga (Sue?) then farm the things Misery spawns from health, then when I'm full, slowly and carefully kill her. Then the core is pretty simple.