Grand Slam

Loot, craft or trade for grenades - regular normal-looking frag grenades are fine - then look for a group of enemies close together.

There are a couple of ways to get enemies to bunch up; e.g. patrols often have 5+ enemies walking in a line, but when they stop and rest or change direction they'll often bunch up really close and a grenade in the middle will do the trick.

Alternatively, on your first exit out of the Interchange to Slatten Dale, there'll be a group of enemies in the tunnel close to the entrance. A grenade into this group - even if they've already spotted you - can also knock them all out in one go for this achievement.