Pyromancy is pretty beastly. I have my flame at +11 and was able to 3 shot
with Great Combustion doing over 400dmg a touch with the Bellowing Dragoncrest Ring equipped.
I went in prepared for a long, drawn out fight because I'd heard how tough and durable he was but he ended up dieing really quickly to the pyromancers flame. I really had no intention of using Pyromancy as anything but a support for my up close and personal playstyle but it's so strong it's sort of silly not to use it to it's fullest.
Is Firestorm as good in Dark Souls as it was in Demon's Souls? I think I remember killing Maneater in 2 casts in Demon's Souls with my Royal and just being shocked at the destructive power.