I wouldn't worry too much about clearing the A12S dummy. Like others have said, it's a poor indicator for tanks in general because you'll have Grit on the majority of the fight. You also have to factor in Reprisal won't proc nor can you use Blood Price. Both are sizable DPS losses for Dark Knight, especially if you're closer to the minimum ilvl. As the MT, you're only concern should be aggro for prog. Worry about optimizing your DPS after you've gotten comfortable with the fight.

Way I would open: Plunge -> Dark Arts -> Scourge -> Carve & Spit -> Hard Slash -> Salted Earth -> Syphon Strike -> Delirium

This way, you're weaving ogcds in between GCD delays. It'll let you squeeze just a little bit more DPS. From there, transition into DA + Souleater combo and pop all ogcds as they come up. Make sure you're keeping Delirium and Scourge refreshed.

That all said, if you're more comfortable with Paladin. Stick with it. Although, Dark Knight may be optimal, Paladin can clear A12S without any hurdles. You'll only need to switch up a few mechanics-- specifically WAR needs to do the Time add.