Just wowhero'd myself to see what my last rank on Durumu was. Still a top 5 and i havent seen the boss in a few weeks (due to extending H Lei Shen which finally died a few nights ago).
Ended the fight at ~325k dps.
If you are going for ranks, you will need to be a full mastery build. Dont switch to the little adds, thats pointless they die to passive damage anyways. Do switch to the fogs as needed / if they are in range.
I personally wont go into my beam assignment until after the walls are down. Cant be chain lightning spamming and moving around
When the walls come up (with the death beam, as the maze phase starts), i will aoe on the first wall. After that wall is down, i pop Ascendance + Spiritwalker's and run the maze spamming lavabeam on boss til the rest of the walls are down. As long as you have 4pc you will be able to Ascend + Spiritwalk for every single maze phase.
Rng factors on this fight for ranking include things like... did you get beamed and forced to move? Red beam while icewalls are up killa your dps. Did you get targeted for Lingering Gaze as Ice Walls come out? yep, that sucks. And then it depends on how many locks / mages you run with. More locks you have the less cleave time you will get.