Improved Fire Nova Totem imo. Having an AOE 2 second stun is really handy :P If you time it right you can stop them before they get to setup their aoe/silence etc. Make sure thunderstorm is setup to be used one at a time (round robin) and all at once. Being able to smack people away while stunned (as fursphere said) is really going to help you.

Positioning needs to be looked at. And you really need to spread out more. (As others have stated, just backing them up hehe)

Pretty much just practice.