Ok. first thing is to spec talents so you get "Combustion". Put combustion on and go and make 2 fire crits. The spell critical chance will stack. Log out. Put combustion on again. Go and make 2 fire spell crits. Repeat until you have a 100 % critical chance on next 3 fire spells. Go and reset talents. Spec onto whatever you want but dont fill in combustion. DONE! You will still have combustion on yourself and have 100 % crit chance on all your fire spells ! This buff goes away when you die or get dispelled