From Wowwiki (re Olm the Summoner):

[quote:f70ba63e90]He should be tanked by his own Felhounds via Enslave Demon. Olm will place a stacking debuff on the Wild Fel Stalker that must be removed by the pet, which is an ability that is not auto cast but on a short cooldown. The Felhounds also come with taunt. The Summoner can be tanked with 2 Felhounds or 1 Felhound and a tank. Additional Felhounds must be Banished or enslaved by remaining warlocks. Not stunnable.

If two tanks are available to tank Olm, having them tank is also viable. A warlock is still need to Fear and/or Banish the Felhounds. The benefit of this strategy is that it is a bit less chaotic and not prone to spell resists that can happen with a warlock tanking Olm.

ALERT: As of patch 2.1.2, he can now death coil the felhounds. Unsure if this is a bug or not. [/quote:f70ba63e90]

So basically the warlocks enslave the felhounds and use them as tanks while others dps him down. Since we've got a bunch of 'locks, that sounds like the best idea to me. Seems like the Fel Stalkers work well as off tanks for this too if we've got extra 'locks or as the felhounds die off.