I think you have his skill wrong.
It does damage to all enemy units based on 50% of each card's MAX HP, so it's not random at all.
So, for example, if the opponent has a ranked up Dual Axe Wielder and a Samurai Girl on the field, it does 45 damage to Dual Axe Wielder and 20 damage to Samurai Girl.
Silly Dean -o-
Cute dog too cute...