With the proper gear every heroic can be AoE'd. I aoe tank every heroic and all of karazhan with zero cc. Which heroics can be aoe'd at what gear level is more of a feel thing. Mobs that make it harder to AoE are casters, fear mobs, and mind control mobs. Depending on how many there are in a particular pull can make all the difference.
The tank just has to feel it out, it will probably result in a wipe or two, but it is part of the learning process. It helps to have a healer that you run with all the time also, the two of you can discuss how pulls went, and adjust how you want to do the next pulls.
For example when I first did heroic slave pens in all blues and a couple epics I had to use three cc for the large pulls between the second and third bosses. In gear slightly less than what I have now I was able to AoE everything, because I can soak the damage I take from behind while feared without much difficulty.
Another example, when I first did heroic botanica the three man pulls with a mender and two others I couldn't aoe tank due to the instant nature damage attacks they do. It was just too much. Now I pull the room with Laj in it and the room before it by the room.
Just talk with your tank and feel out what you can and cannot do as you two both gear up.
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