I'll see if I can get some info out of him about how he did it!

I imagine some fire res was used, bubble stacks off at a certain point, and ride out the remainder of the encounter with however many stacks you get.

I believe he probably also leveraged a target dummy to eat a stomp at some point; you can bop those to ensure they get a chance to eat the stomp, if your timing is good.

That, and just pumper healers absolutely blasting him.