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. :P Tested several times.
Second exploit. With combustion on, cast a spell such as fire ball or pyroblast. When the cast is done, and the ball is heading twards the enemy, fire blast. If all goes well, they should both crit. I use then when my combustion is on its last charge so i can get an extra crit.