As many other have done, I thought I'd create an account on here to say many thanks for the awesome guide, Ackthal... it most definitely has helped me a lot! After around 50 or so attempts I finally got him down the other day, woo hoo, I should be green fire enabled now.
I say "should" be green fire enabled now because, well, I killed Kanrethad and successfully completed the "Infiltrating the Black Temple" quest at the end but I'm still throwing out the boring orange fire. I seem to have encountered a bug where after completing the quest nothing else happens. I stuck around for like 10 mins a