Ok, this one is debatable whether it was actually a critstab. Assuming it is, here's the damage breakdown as I calculate it:
The melee damage and crit stats as of the moment of this fight, including the 3 stews, resulted in
12190.237 melee damage
78 crit exactly
A basic critstab here would be:
12190.237 * 78^2 = 74,165,401.91
I made a macro to do some animation canceling (I think that's what it's called) and managed to cast darkness, throw a grenade, switch to my revolver and fire, then switch to my umbrella and knife in about 1 second. This allowed me to shove as many charm bonuses with short durations as possible onto the main hit.
Duelist grants x3 damage, ice bullet grants x2 damage, darkness grants x1.2 damage, and Magwytch is weak to piercing giving x1.5. This results in a pre-charm damage of:
74,165,401.91 * 3 * 2 * 1.2 * 1.5 = 800,986,340.7
I had the following charms:
Charm of Morale
Charm of the Spellsword
Charm of the Arsenal
Charm of 100 Blows
Charm of the Juggler
Charm of Riposte
Charm of Resurrection
Charm of Called Shots
Charm of Rage
Charm of Venom
Charm of Fatigue
Morale, Riposte, and Venom weren't active at the time of the strike. Spellsword, 100 blows, juggler, and fatigue are all additive with each other. Arsenal doesn't appear as a buff, but it lasts for about 5 seconds, the countdown for it would begin on the last hammer strike, and 100 blows decays at a rate of 4/3 stacks per second. Considering that after the strike I was at 17 stacks, then this indicates that the 16th stack had not finished decaying before the strike, and no more than 3.75 seconds had passed between when Arsenal was primed, and the strike, so I would argue it was active.
Arsenal grants x1.5, called shots on a critstab grants x1.5625, and rage grants 1.75 at very low health. Spellsword grants 0.5, 100 blows grants 0.05 per stack, juggler 0.25, and fatigue 0.15.
800986340.7 * 1.5 * 1.5625 * 1.75 * (1+.5+16 * 0.05+.25+.15) = 8,870,297,953 damage
................Allegedly