boss spamming skill

I tried to kill the breach boss to get the last 2 points and when he was low on health he started spamming the skills, he didn't finish one and was already using another. How am I going to kill the bosses like this? Do I have to create unbalanced classes just to be able to kill the bosses simply because my class and build don't explode the boss in 2 hits? I have enough damage to kill the boss but I can't hit him because he keeps shooting skills non-stop.