Right at the start of the third mission "Persona Non Grata." Right at the start of the mission, Price will tell you to get on the balcony to the right. Do so, and look down onto the courtyard and look at the big double doors at the back.
As soon as your guy down on the ground fires a rocket at the helicopter, cook your grenade and aim for the doors. A second or 2 before it explodes, throw it. Just as it gets near the target, 5+ enemies will breach through the double doors and your grenade will welcome them.
- #2: The second place to try this on is "Back on the Grid." As
you make you way up to the church, don't climb the last set of steps. Cook a grenade and then walk up. As you do, the double doors will swing open and you'll be greeted by half a dozen guys. With your cooked grenade, chuck it at them and you should kill them all in one go!
- #3: Finally the third spot is on the next mission "Mind the
Gap." Just play through the level until you open the truck doors. You'll now be ambushed. Deal with the onslaught, and make your way through the small warehouse and out the back. You should now go through a wired gate and need to head left.
Keep to the right side and get down to where you get blocked off by a brick wall and 2 portable toilets. From here start to cook your grenade and now look across to the left corner and you'll see loads of guys run out and all group up in one place. Quickly throw your grenade into the group to unlock the achievement. If you're not fast enough, the helicopter will do a strafing run and kill them all.
If by chance you miss or the grenade explodes too early, simply restart from the last checkpoint and try again.