I see Atomos hasn't been mentioned. I'll explain him.
He takes 30% of the monster's total health plus the number of amethysts you have. So if you have 70 amethysts then he'll take 100% of their health. Obviously, he can't so more than 9999 . But still 9999 every time isn't too bad.
If you only get the short animation the formula is a third of that. So 10% plus a third of the amethysts you have %.