First thing you need is a mage with combustion, obviously. So say your going to AoE fire spec, and yes, you can. You get like 4-5 mobs, and then pop combustion. Now when you use an AoE fire spell, if one crits, it should crit on all the mobs, not just 3. Yes, i know it is possible it can crit even without combustion, but when it crits on all 6 mobs is very unlikely that it would happen. Tested several times.