The highest I've ever seen is 299970 - 30 9999s with the Rezoluken-Lionheart combo - 8 hits with Rezoluken, 22 hits with the Lionheart.

Sorry, but I've yet to see anything go past 300,000 dmg. Anything over that IMO is overkill. The highest summon attack I've seen is Eden's 60,000 dmg.