The team setup you are playing against is a 2345. They aren't prancing in at just any given time, they are dropping your groundings as soon as your Shaman does so and rushing in before your cooldown is up. It's a premeditated strat I've dealth with many times
Their burst is insane bust it's also gimmicky, you have 2 options you either blow up a target before they blow their cooldowns or attempt to survive the burst/cooldowns and then unload.
The best targets are really all 3 DPS, the Warriors / Mage / Shaman are all good in this scenario, I usually purge the Warrior as he runs in and then switch to a caster and purge them. Then I drop grounding and either heal if needed or unload on either target, switch and blow your cooldowns on the other.
Keeping constant pressure on the Mage is most likely the best option though - I ran into issues with this setup using a Shaman for a heal and the poly is disastrous. If he Iceblocks, just heal through the other players damage and then blow your cooldowns the instant he comes out.

In final, survive the initial 10 seconds of the fight and you win =D