Team Cleric

I went through a lot of wipes with Alshain and Co. on this today but I think we hit an awesome strategy.

Alshain (paladin tank)
Ama... something (frost mage)
Fig... something (resto druid)
Nil... something (enhancement shaman)
Lesasia (blood dk)

Mark everything except watchers. Only kill things which are NOT marked.

The sekrit is the druid. Roots. LOTS of roots. On the skirmishers.

For each group:

Root the skirmisher (except the right side, which is warrior and a caster). Death grip the other one and kill it. On the caster/warrior pair, root the warrior. Also, Avenger's Shield is awesome for this because of the Silence effect. We got the gate keeper down to 20%, so I think this can work.

From what I remember when I was keeping up with reading stuff, the most effective and reliable strategy for that fight basically looks like suicide bombing.

Get a *lot* of DPS, all of it as single-target-heavy as possible. Get a heavy-geared tank with good defensive cooldowns. Buff up like crazy, pop heroism, and charge. AoE tank the entire room, with Shield Wall, Shield Block, Last Stand, everything you've got. Burn the living *crap* out of the Gatekeeper, ignore everything else. Gatekeeper dies, tank dies, everyone wipes. You're guaranteed to die for attempting it, but if you have enough DPS you all die with the achievement.