Achievement text: "Kill five enemies with a single 20mm HE grenade."
A good place to do this would be in the MJ12 Hong Kong base at the start. Pepper-spray the group, then boom. Another spot might be near the middle port side of the Wall Cloud after starting the scuttle sequence.
I got this by accident in Area 51 Sector 3. After meeting with Helios, I took out the initial group coming from the upper left, then peeked out the far left to see a lift with mooks on it. I fired a grenade at them the moment it stopped, and it popped.