Originally Posted by
katmage
Combustion damage is low. This is what I've heard re: tactics - combust on the first door adds (when all the balcony adds are down) and spread. For each following door, combust on Horridon (since he takes 50% more damage) and spread to the adds.
What I personally do is use combustion on the adds every time all the balcony adds have dropped (since we have Horridon tanked quite far, and it's hard to spread to the adds), and combustion was my #2 damage dealer on our kill this week, with average ticks of 32k. Looking at a couple of attempts with your mage, his combustion is #7 damage dealer with average ticks of 7k-18k.
So what you want to do is find out how he's using combustion, and also ask him to build a bigger ignite before using combustion.
I'm also going to third what some other people have said here with Nether Tempest > Living Bomb.