You Runed Everything!
When Draw Power begins in phase 2, runes on the floor light up in an order, and the incoming adds must be killed in the same order they lit up. Once this has been done, the 'Runic Mastery' debuff will appear, ensuring the achievement is earned once the fight has finished. It is best to pop / after the debuff appears to make sure Draw Power doesn't happen again.
- Patch 7.1.0 (2016-10-25): Added.