This is the art of multi-bxoing, figuring out how to synergize your different characters.

I have a DPS spam button which is mostly direct damage spells, stuff like Shadow Bolt, Fireball, Frostbolt, etc. I have a 2nd DPS button that I use for DoTs and debuffs etc. This allows you to hit your 2nd DPS button 4 or 5 times to get all your dots up and then you spam your main DPS button till the DoTs drop.