1º of all you dont need to be DRK t clear As12, so dont change to a job you are not confortable to clear that turn, but well if you still want to improve you skill as DRK sure.

-darkside up alwais.
-scourge alwais up on the target and shalted earth too.
-soul eater is you best dps combo with dark arts, delirium is the second without DA, switch betwen this 2 to manage you mana.
-use all you ofgdc, plunge, low blow, dark passenger and carve and spit+DA.

SSS is bad in the fact you never use grit on it, the dummy only check you dps rotation so is very poor in that way for tanks, and DRK is a job how dont have rotation at all, all depend of how much MP you have and find the way to get it to use more soul eaters with DA.