should've said twelve-powered apoc which could take down marvel-verse all earth heroes for millenia. Without such apoc has been low on feats.
Only recent offpanel feat i remember is
Unless I misremember dark beast said apoc took down a celestial offpanel in the age of apocalypse timeline(x factor comics reference).... note that that apoc didn't have twelve merger tech, afaik.
Taking down a celestial is no mean feat.
I'm actually leaning towards Thanos/Apocalypse. If it was a h2h brawl even moreso. People will argue against it (KMC's Anti-Apoc Meme/tables), but I see Mongul as the bigger weak link here than Apocalypse is, particularly considering Apocalypse's versatility and the fact this is a fight and not a long drawn out plot to be thwarted by mutant heroes at the cusp of victory.