In Mission 9: The Fold Weapon, you will get a new Titan with a minigun. It has the "Smart Core" ability, which automatically targets enemies. It also shows you the amount of enemies in the current area. After fighting a bunch of Titans, you will go through a tunnel with 30+ enemies. Wait for them to group up, then use Smart Core to easily get 25+ kills in 2 seconds and the "Annihilation" achievement. Just stay at the opposite end of the tunnel so all enemies are in your line of sight. If you fail, You can restart the checkpoint to try again.