To perform a melee attack, you must press . Run up to enemies while playing in Quick Matches and try to get kills this way. You must be close enough. Hitting them from the back is your best chance, so try to follow enemies through portals and surprise them from the back. This is a fast paced game, so you will likely die many times attempting this. This achievement requires 3 melee specific kills in a row, and is only voided if you kill an enemy by shooting them. This means dying is fine in between.