In order to get this achievement, you need to be playing as Titus and get kills while his Y ability is active. To play as Titus you must be the host of the lobby. His ability gives you a temporary stat boost.
A quick method I found for this achievement was to select Kadaku and play the second mission called severance. Right from the start you walk forward and there's two groups of tyranids. What you want to do is throw a grenade at the left group, activate your ability, then throw another at the right group. After that just shoot whatever's left until your ability runs out. Once it runs out press start and reload checkpoint. Rinse and repeat.