Against who is the clanboss using his stun attack?

Hi, I have a big question. I'm struggling to find out who the clanboss's attack priority is. I thought he attacks the one with the lowest HP, but he is always attacking my healer and that's why i can't reach the max amount of damage on him. Is it the affinity? Because my healer is a void champ and the rest are magic. Or is it because the healer is the only one who doesn't do any debuffs on him? I thought of everything but i didn't get any further... I did some research but couldn't really find anything... help ;_;