Just gonna make a post that i don't believe any1 has done before. I dont know all the good combos but i'll share the ones i do know.
Prophmage - First use Boiling Blood then use Ignite then cast Incinerate (AKA Fireball) OR you can cast Erupt instead of fireball, now finally cast Despair
Ranger/Prophranger - First use Haemorrhage then cast Envenom then use Crystal Blast then use Tornado then cast Despair then finally cast Blazing Arrow
Sinwar/Sin/Prophsin - First use Eclipse then cast Jolt then cast Frozen Spires then use Lethal Concoction then use Rupture then cast Sear and finally use Torture
Sorry i dont have a combo for all the classes and the Assassin ones arent the best but they have always worked for me. Feel free to post better combos and combos for classes that i havent posted.