Tearing my hair at difficulty

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Also remember that the enemies will try to guess where you are shooting from even if they can't see you! The AI WILL fire at your general direction upon being shot at, sometimes even at a completely random direction. But the point is the AI tries to guess where your mercs are, so they try to flush them (you) out. If you have silencers, you can fire a shot or two at the enemies during night, stop, maybe move to another spot, and repeat. They'll have much more difficulty tracking you down.

I took out the entire rebel camp with just one merc at night with a silenced 6.8mm H&K DMR rifle. I fired one shot every 30 seconds maybe. They were just running around like scared sheep, some of them firing at my general direction, but they never could pinpoint the location of the merc.

Also, early in the game, use shotguns - you're much more likely to hit something with them. And don't be afraid to empty an entire clip from an SMG.