Not a lot to do about it.

If ns+em is up then retarget and waste them.

Other than that I can only think of trying something like dropping an earthbind , frost shocking, and backing your shamans out of there so you can apply a nuke.

Another alternative is to have an emerency earthshock rotation macro. e.g the press once, shaman one hits it., press a second time and shaman two has done it and so on.

You may even have the other shaman lesser heal themselves (perhaps even taken the talent in resto that reduces your chance of interrupts when healing.

Perhaps 5x fire nova totem, when you see him running in - or aoe him back with 5 sets of magma totem.



Other things I would need to check out - are there any potions that give you the ability to cast uninterrupted