Whenever I play as my shaman team, I generally rely heavily on my Magma Totems. 4 of those does a dangerous amount of damage. I keep all five Thunderstorms as a kind of, "Oh Crap" button, and after five of those, you can generally pick them off with a second volley of Shocks. Even if you don't Thunderstorm them, with 4 (or 5) Magma Totems, if you just outheal their DPS, the totems will wear them down. I'll Magma Totem and then Shock them. Do a chain heal, or wo, then do another Shock. It's usually game over by then. I've had multiple people run up to me at once, just to get mowed down by those totems, while I stat there casting Chain Heal x5.

You allies also like to huddle around the 20 totems, as they see you as sort of a walking safe haven, so you're generally not alone for took long (if ever). I have people specifically follow me around in WG just to provide support and keep themselves relatively safe.

If you don't want to blow all five Thunderstorms at once, however, I'd suggest the Thunderstorm Round Robin, like you mentioned, but instead of wasting the four other cast potential, to maybe throw in a Frost Shock and a few Earth Shocks, too.

/castsequence reset=45 Thunderstorm, Frost Shock, Earth Shock, Flame Shock,
/castsequence reset=45 , Thunderstorm, Frost Shock, Earth Shock, Flame Shock
/castsequence reset=45 Flame Shock, , Thunderstorm, Frost Shock, Earth Shock
/castsequence reset=45 Earth Shock, Flame Shock, , Thunderstorm, Frost Shock
/castsequence reset=45 Forst Shock, Earth Shock, Flame Shock, , Thunderstorm

You'd have to target them however you preferred method is, but at least this time, each time you press your "Oh crap" macro, you'd knock them back, slow their movement for a bit, and do some extra damage. Four of Five shamans would be doing at least some sort of damage each time you pressed this, while still only blowing one Thunderstorm cooldown.

That's all hypothetical, though. I've not actually tested it out, anyway. I'm more of a 5 Thunderstorm kinda guy, myself,a nd that generally works fine. Only if I'm deep into enemy lines and get real outnumbered do I tend to get overly bogged down with multiple melee.

Have fun!