Alternatively, if you are lucky and have a few backup saves ready, try loading them one by one and traveling back and forth from lower city until you find one that works. Once u have done this, check all the fights you had in between that save, and the next save that doesn't work. Should reduce the "undead" body search area considerably... Try to think of it as a murder investigation sidequest in order not to break your keyboard... XD
Otherwise, if you really can't find the "undead" HP positive body just start playing from that last working saved game that you have (really frustrating I know). And obviously always check body HP after every fight.
