As far as warlocks go, if the warlock in question has 2/2 Intensity and a paladin (for concentration aura) is available, constant hellfiring is the way to go. We found it a much easier way to keep the imps controlled then SoC spam.

I have mage tanked the imps in Illhoof more than a few times with 5/5 Magic Absorption and max FR. Here are some things I learned for mages interested in doing this.

Use Dampen Magic or Improved Dampen if you have it. The net incoming damage per imp is about 20-25 per fire bolt, which is trivial to heal through even with a room full of imps unless you get the Amplify debuf from Kil'rek. I made a point to call out for extra heals when I got the Amplify debuf, as the incoming damage better than triples for the duration.

Start out with max rank Arcane Explosion to get the attention of the first few imps, but as more imps spawn start to downrank. With a room full of imps, healer threat is divided up among them all and it only takes a rank 1 or 2 AE to grab and hold the last few pairs of imps that spawn. After that, low rank AE periodically to stay ahead of healer threat. The raid should let you know if an imp or two breaks loose from you, since it's hard to tell with a screen full of fireballs and scrolling combat numbers rolling by.

Mana is a non-issue. I was even able to keep focused on Illhoof himself and assist on the chains as needed. However, the Lightning Capacitor does not hit Illhoof from AE crits on the imps as suggested and indicated by the tooltip (it always hit the target that proc'ed the 3rd crit and not your current target). I barely needed mana rubies, but it is possible by popping Fire Ward and getting a priest shield to evocate to full mid-battle.

No great amount of HP is needed. I think I did this with less than 8k most of the time, as long as your chain healers are sharp. You will get chained eventually as the mage tank.

This is by far one of the most fun fights for me as a tanking mage and I highly recommend every mage try it at least once if you have the FR and spec.

The cooldown is there so that if a mage is AEing 10 mobs, he doesn't suddenly get a massive mana boost, instead only gets the mana from a single kill.

I have killed illhoof with mages and locks AEing (different raids). With the lock it is simple, just Spam SoC. Destruct locks with the interrupt talents can get a concentration aura and spam hellfire (doubt this is needed now with the reduced hp though). With the mage we had a pally healing with RF and when imps got a bit crazy we just sent the mage in to splat them all.