The only trash in CM that requires any strategy is pulling the big mechs back to the cannons. Usually the tanks take turns so they can keep up a steadier stream of adds, and the rest is just a matter of remembering where the cannons are. The rest of the trash is pretty much just AOE spam-fests. Follow other people to try and skip the skippable trash, be ready to grab it when someone inevitably gets spotted, and tank what you can.

On bosses, if you have time to safely DPS, do it. You can at least contribute Storm's Path and/or Eye between add waves. Prioritize being in position for the adds you can be in position for, though. Until you're comfortable with the timing, get to your spot early and wait.