First of all, I searched but did not find any threads like this.
This is only applicable for Dps who are not completely needed.
On the last boss in the Nexus on heroic, if you stand still you get a debuff called Intense Cold. To stop this you jump or move around. You cannot allow this to get above 2 stacks.
I found that if you stand a little bit outside of the doorway before she frost nova's you do not get the debuff Frost Nova rooting you to the floor. After she yells, run in and dps for about 10-15 seconds, then run out again. Repeat until dead, and you will get the Intense Cold achievement.
Good luck! :wave: