I found a great (situational) use for Combustion last night... I wished I had put it in my female Warlock's book. I was in FC and opponent new Warlord was building up in his starting corner. He had multiple Dwarf Panzerguards to keep his Mage and other things in that zone from being attacked. I had a fair amount of damage on the Warlord plus six burns by this time.
Out comes Renewing Spring and the healing wand. I can't get to the Spring, and the burns start coming off when he drinks... a nice Combustion at that point might have done around 10 dice of damage.
On another note, I found a use for Gate to Hell that game, and even opened it in FC.