Not sure of the source, but the most powerful attack the Emperor ever pulled was a supernova-strength psychic blast he used to kill Horus. The attack required all of his energy reserves and was what resulted in him ending up in a coma, though he was already pretty busted up from the fight before hand. Anyway, Supes has survived plenty supernovas before, so the Emperor won't necessarily be what kills him, provided you don't classify Psykers as Wizards. If anything Chaos will finish him.