Help doing dps as shadow

Hello I usually play heal with my priest, I need to get some cinder for my legendary, so I went to Torghast and it feels like you are pretty squishy and you don’t do damage outside void form.

What I do is cast vampic touch(misery talented), mind blast then mind flay, it takes forever to kill even a mawrat, any suggestion?

Thank you

Reroll. Shadow is literally that bad.