Well, be sure to all use purge at once and clean off a lot of their buffs if you're really having trouble. Sounds good in practice,and is, but I know it can sometimes be hard to cast a non-damaging spell.

Put wind shear on each of your characters set to fire off before a different one of your dps spells. eg. My main always fire off wind shear rite before he shocks, next toon always does it rite before Lightning bolt, next one rite before chain lightning (providing you us it a lot, which you should be), and the last rite before lava burst.

This allows a mindless way to always keep interrupting your kill target. Keep your round-robin wind shear key available for the times you see them casting a spell but don't have time to attack, and spam it until it fires in case the first few shaman have it on CD.

To set to fire off before:
""
#showtooltip Flame Shock
/cast Wind Shear
/cast Flame Shock
""