This one can be tricky, but your best bet is probably the mission Submerged as there is a (glitchy) boat early in the mission and lots of enemies. You may have to restart a few times because the boat you need tends to get stuck.
The mission starts with you parachuting towards an enemy encampment. Clear this area out and then set the first charge in the first bunker. Stand on the cliffside and you'll see a dock below, along with a large group of enemies approaching from the road. Don't shoot these guys, as these are the ones you need to kill when your boat arrives.
Now here's the tricky part. An enemy boat is supposed to come land at the dock, but it tends to get stuck on a patch of land on its approach. This delay can cause the enemies to run out of your range and possibly get killed by the rest of your squad.
If you see the boat get badly stuck on its way to the dock, your best bet is to restart the mission and hope it takes a better route next time. Try standing in several different spots after blowing up the bunker; such as on the cliffside or down beside the dock.
Once the boat does actually land, just shoot a lot to draw the enemies towards your boat, and then pick them off along with the other enemy patrol boat. When the boat does arrive in a timely fashion, you'll have more than enough enemies to get this achievement.
Remember, while you'll unlock the achievement immediately, make sure you DON'T QUIT once it unlocks; either finish the mission or just fail...if you quit, it won't save the medal towards your campaign progression!