I use full block set in heroics but got a tip or two for you nonetheless, I think ive done COS 14 or 15 times.
Cleave, cleave, cleave
Use charge + intercept on trash zombies to get ahead of your dps and buy some agro time, and also to build nice rage amounts. Dont even bother using any abilities on zombies, Dshield + thorns/anything will kill them by itself.
As for packs, I first bash the necros, round up then work on the rest. Don't even shock right away, you can save it for when one ghoul or something goes for the dps folk.
Dragon packs same, target Agent as quick as you can and start off with silence, works great for me. Dont forget to use both bash and gag throw to make gathering them easier, especially in the hallway just before market row, I usually do TC on the pair of melee, charge to agents.
On the last part cliick target the ghouls/aboms/spiders as tabbing or anything else is unreliable with the zillion zombies. I pull them to the spider as that one stays stationary.
Keep practicing and youll never do another COS without killing timed drake (:
