I think this is very easy, but some newer raiders (and even experienced ones /sigh) can have trouble judging where the "invisible line" is for the sections. I took four flares and put them right in the middle of each section, worked like a charm. Also, we 5 manned him in the 10 man version, and that was with 1 heal 2 tanks (both in tank gear) and 2 dpsers. As said he does no dmg and you can killhim easily if you don't die to the eruption.
Edit: 5 man was from 79% to death.

