Don't remember the end of Prototype too well, but I think Mercer absorbed a crow that swooped down to eat at the little pile of goop that was left of him to start his regen after the nuke.
Tentatively gonna say Lord Foul. I don't think Mercer can bring him down and I feel like Foul can eventually at least incapacitate Mercer.
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Gamertag-Murphiroth
Pokemon Black-1978-9490-1161