He already did once, Asterius is the resurrection of Bells lvl2 minotaur fight. Monsters that die resurrect/respawn in the dungeon over and over, some are just created from nothing (I guess) and some of the ones with significantly unusual interactions tend to be resurrected with fragments of their past lives, hinted at by the things Wiene says about how she has/had strange dreams, and considering that Asterius also used his horns like in Bells fight during the Ais vs Asterius fight
the way they showed Aiz cut Asterius arm like she's stronger than him when in truth she only caught him off guard that's why she was able to cut it, the way they'd portray it here in anime kinda sucks just for Aiz hype, Asterius was a Lv.7 Xenos trained by the strongest adventurer in Orario that's why it will take all the Lv.6 of Loki familia to take him down.
800 years since she first learned the spell. Assuming Fels is using the spell immediately after someone near her dies, then Wiene pretty much won the lottery with something like a 0.0000001% of success