He's probably used some kind of magic to stop time on her body or maybe he froze it? Because the problem with ressurections is that if your brain doesn't have oxygen, that is only provided when you are alive, then it dies and brain function ceases to exist. So he had to stop her brain from dying in some way by stoping the flow of "things" in her body. It is possibke with magic I guess? We don't know what she died from, but if he can't ressurect her even after all that, then it must be a chronic disease or something realted to mana, which is why he wants/needs the mana from an elf. Maybe the reason why he considered making a deal with what I assume is a vampire is because there is mana in fresh blood or something like that, so if he keeps replenishing her blood with new one, maybe it would work? It's the only reason I can imagine, so it must be a mana related disease or a curse?
This is why in the Frankenstein movies they always make Frankenstein dumb, because there was no way for an alive and perfectly well brain to ever be used, at least this is what I assume the writer of the story thought xD
Won't it be too late now??
Usually the permissible time range was immediately after death, no?