Under the Fire tree you’ll see a talent called Ignite which applies a DoT to the target after it receives a critical from a fire spell. The DoT is 40% of the spells damage over 4 seconds, but here’s the important thing…
If you crit again within those 4 seconds while Ignite is active it’ll reset the duration AND stack the new spell onto the DoT. What’s even sicker is that all Mages can contribute to a single Ignite effect - that’s why you see videos of Ignite dealing thousands of damage per tick.
First mage to start this rolling ignite gets all the aggro from the boss. And will eventually be a target, but damage can be so huge that it’s worth it.