To people having trouble on this one:
I did this on the first 5 enemies there are. Pop the first dude on the cliff in stealth, no questions asked.
The next two guys are about 40 meters ahead - stealth over to the right and prone in the bushes where you can see just the guy on the right. Pop him while prone. The other guy won't be alert. Restealth, and kill the other guy.
The next two guys are after the short cutscene with Aztec. Just basically rinse and repeat the strategy from the previous two. Wait until they're a short distance apart, and while prone AND stealthed, pop one, then the other.
**MAKE SURE TO STAY STEALTHED AS YOU SHOOT!**
Yeah, you'll lose all your suit energy. It doesn't matter. As long as you're prone on the bushes, they won't notice you.