Kill 4 enemies simultaneously with one explosion (Single Player only).
This can be unlocked with any weapon or equipment that gives of an explosion, so don't be nailed down by one thing. Personally the best is the RPG-7 which can be found at any store under the Launchers menu or obtained for free by unlocking Radio Towers. Now for the best chance at getting this, head to any un-liberated Outpost. Once you arrive, make sure you get caught and the enemy NPCs set off the alarm. Now wait out the battle until the reinforcements arrive. Keep an eye on the map of the location they're coming in from and wait there. Once they arrive, it'll be a convoy of 2 vehicles and it's the second one you're going to want to shoot with your RPG-7.
If you get the hit, the vehicle holds 4 enemy NPCs and the they'll all die and the achievement will unlock.
Get an explosive weapon from a weapon locker or general store. The RPG-7 rocket launcher works best. You can either buy it or activate radio towers to get it for free. Now go to the Welshore Wrecker's House Outpost (coordinates X: 437.9, Y: 795.0) and make some noise until the enemies activate the alarm. Wait for the reinforcement Jeeps to arrive, but keep at least one Outpost guard alive. In the second Jeep (the one without the turret) are usually 4 soldiers. Shoot this vehicle with your rocket launcher before the soldiers get out. Alternatively you can also plant some C4 and group up some enemies, then blow it up. If it doesn't work on your first try simply get yourself killed and you should respawn somewhere around the Outpost and can try it
Yeah I got this for some reason late in the story, many times I had killed 3 people with a nade but the 4th some how survived.
I jut did this though easily when I was in a Bunker (one of Hoyts missions) and I don't know if it was ment to be or I got lucky but 5 AI's stood still in the same location next to each another so I just cooked a Nade, tossed it and boom achevo.
Very simple way to do this: later on in the game, after meeting Sam, you play a mission where you have to zipline back and forth over a gorge and then pick up an RPG to blow up two cars on a bridge. Don't shoot the cars directly. Just shoot the RPG right at the middle section of the bridge and it will blow up both cars and all the people on the bridge. Pop goes the cheevo.
Gotta revoke my upper post. Cheevo isn't glitched. Just got it a few minutes after my post. Maybe it didn't work since i am/was wearing the privateer outfit and these "enemies" in that mission (also privateers) weren't counted as enemies.
Got it liberating an outpost by shooting 4 guys with an RPG.
Do what Cougar3268 did. It was super easy! Go down to one of the Trial of the Rakyat near Badtown. The Transport Destruction trial makes you use the RPG-7. Don't shoot until the second jeep comes down from the road in the middle. Then, wait for the jeep to come across the guys on the left and BOOOM! Achievement has been served...