1st, make sure your weapon (at least) is on par to your level. Then, as you're running in, spam tomahawk button until it goes off to initiate the pull. Then, when the mobs are gathered, pop a flash, then move through the mob to make sure they 'stack' throw out an overpower or 2. with 3 mobs, I usually overpower twice. Then start in your HS>SS combo on the mob the DD is hitting (or whatever you mark as 1, if they're not hitting #1, let them have it lol). Then you can go into maim combo and skull sunders rotating on individual targets.
(just for clarification, in my last sentence, by skull sunders, I of course mean combo, not just skull sunder by itself)