I was in this game, but I was not the player in question.
Eighteenth level Fighter/Scythe Master
Made a five-foot step and then full-attacked a creature while hasted. With class abilities, he needed a 16-20 for a critical threat.
The first hit was declared to be a Ki Strike (I think that's the name... max weapon damage of 8) and was a critical that he immediately used the "Increased Multiplier" ability with. The second shot... was also a crit. The other two both hit. The hasted attack was also a critical.
I'm trying to remember... it was a +5 Keen Icy Burst Scythe and his strength was 22 (16 STR + 6 Item). So his damage modifier was +9 for the weapon. He was also specialized in the weapon.
First hit: 120 + 1d6 + 4d10
Second hit: 80 + 8d4 + 1d6 + 3d10
Third hit: 2d4 + 16 + 1d6
Fourth hit: 2d4 + 16 + 1d6
Haste hit: 80 + 8d4 + 1d6 + 3d10
That's a total of:
20d4 + 10d10 + 5d6 + 312
I don't recall how much damage he really did. I *do* remember that the monster was technically a greasy stain after the *second* hit, but that it was the end of the encounter so he rolled all his attacks.
Pretty much the entire game ground to a halt at that point as we all had to stand and stare over his shoulder at the incredible powerhouse his fighter had become. He was potent before, but all those crits made us all SCARED of him (in character).
Average damage for that monstrosity, by the way, is: 434 damage.