1. I tank 1st and 3rd Doors. I mocking banner the 3rd. Vigilance, Thunderclap the first, heroic leap the adds for the initial aggro. People with Misdirect abilities should be using them them too.
2. Shield barrier them, pretty sure they are unblockable but can't remember. The damage shouldn't be too bad in all honesty that it's chewing through your entire HP pool.
3. At the edge of the semi-circle thing, having people move into a straight line with him if they are charged. Found I can grab the 2nd casts (when 3 mobs are casting poison volleys) with AoE interrupt if they are kept close enough.
4. Both? Mock the third door, it's the only one I found dangerous if they MS the current Horridon tank.