Hmm... I hadn't tried Heroic Strike as an aggro builder - I figured devastatex2 was better than 1 devastate and 1 heroic strike. Perhaps I ought to not be using a cooldown on thunder clap - it may just not generate enough aggro (even though it is hitting all four mobs at once) in place of one more devastate.

The other thing that is tough in my setup is that my DPS is assisting my tank for targets, so the more I tab around to generate aggro on multiple mobs the longer it will take for anything to die, which means the more healing my tank will need, which means the better the chances that my healer will pull aggro from at least one mob before the end of the fight. With my pally tanking I can just concentrate all my DPS on one mob initially, bringing one down quick and simplifying the fight.