Keeping track of damage dealt, Help for a boss battle Options

[Orias Obderhode]

I have a boss in mind for my game that readies an ultimate move that can only be stopped if you manage to deal a certain number of damage to him during his preparation phase. The boss will attempt to use the move several times during the course of the battle.

So the attack pattern will work like this:
"Boss readies X move" (turn 1) - this is your que to attack.
"Boss is still charging up." (turn 2) - if 200 damage is not dealt, proceed to turn 3.
"Boss uses ultimate move - which paralyzes all foes with 100% accuracy and damages you." (turn 3)

The boss won't be too terribly difficult unless you can't manage to deal the right amount of damage or fail to notice that he is charging up.

Is there a way to do this? I'm thinking that somehow you can do it with variables, but I'm just not sure how.

Any help would be greatly appreciated!