In order to obtain this achievement you will have to knock your opponents health down to -5. Your opponent will die as soon as they hit 0 so you will have to do a minimum of 6 damage to them assuming they are at 1 lift when you attack for the kill. I obtained this by building a deck with small creatures and lots of power enhancing spells. I casted spells to boost whatever creature could attack free for the kill boosting their power up to 6+ and swung in for the kill. This would also work just fine by attacking with a creature that has 6+ to begin with.
The online scene for this game is completely dead so you will most likely have to boost this with another player. This will also only unlock during an online ranked match, no player matches will count.