In the Overrun gametype in versus mode, you must play as the locusts. As you get points with damage and kills, you will eventually get 1500 points to unlock the Rager. When you find the humans bunched up together (at least 2), go into rage mode by pressing . Charge at them and try to kill two in quick succession with your melee attack. This is a lot harder to do by yourself, so it might be best to just boost it with 2 people. Have them stand next to each other as the Scout Class and kill them.
Go HERE to find people to help with the achievement.