attack + crit damage i think

Damage Multiplier: 1.7 ATK + 0.1 ENEMY MAX HP 

And, yes, it is affected by crit damage.

Two additional notes to the things krama allready said: it's also affected, like any other attack, by the enemies def, so decrease def will help.

And in dungeon stage 21 and higher, it's capped at 10% of the enemies max hp at all at the boss. Coldheart for Spider and FK 25 doesn't need astronomic crit dam, the damage will not increase any further. In opposite to that, in stage 20 you want as much crit dam as you can get.

thanks guys.  So you're saying the enemy's defense will still moderate the amount of damage done by the 0.1 MAX HP part?

Yes. That's the reason why you should have a team member with decrease def going before Coldheart does her shot.

P.S.

A thread about Coldheart and the damage cap at stages higher than 20 on reddit.