When fighting in a PVP match you will be shown your MVP troop and an icon on the bottom left describing a specific action that this troop did.
I unlocked it with Gorgotha with this team for exploding more than 100 gems!
Gorgotha
Emperor Khorvash
Ragnagord
Kerberos...
So use Gorgotha a few times and win the match und you will get the achievement