Inivgorate -> PSIII if you can before even entering.
Start: Doomspike combo (keen flurry, blindside, b4b doomspike)
CT combo
RoT combo
By then he should be red lol.
Before Hellfire: CT/RoT combo with discretion, use keen on CT.
After Hellfire: I just use Doomspike, will CT combo if a radiant plume positioning allows it.
Haven't done it as DRG in a bit, but I think that's right ><