DoT (Poison, Bio, Dia) and Banish/Scourge + Blood Rite are your main sources of damage, though I would put priority on the DoTs. Just get a rotation going and try to keep at least two DoTs up at a time. You might as well throw in Paralyze whenever it's up while you're at it. Thm isn't about big numbers...but if you keep a close eye on the mob's hp bar, you will see that Thm damage really adds up.

Your main source of damage later will be Shadowsear, but you have a while to wait for that one.