Just inside the room there's a stack of crates. You may want to hide behind them at first, wait for the man on the lower left to move close to the fuel barrel below the ledge, then blow it up (i.e., press Interact when you see the environmental hazard icon, as shown above).
This won't destroy the machine gun, but it should kill the man below and possibly one of the men above.
NOTE: If the man on the left moves to the right instead, rather than trying to blow up the ledge, wait until he's close to the other two men on the lower level and then throw a grenade into the group, as shown below.
The mercenaries will be fully alerted now. So run out from behind the right side of the crates and quickly toss a grenade at the men on the other side of the little metal bridge.
If they don't die immediately, run toward them shooting. Continue forward to the far, right corner of the room, where there's a ledge that will enable you to climb to the upper level. Ignore the reward for now.
Turn left and immediately take out the machine gunner and his companion, if they're not already dead.
When the fifth man dies, 3 more soldiers appear near the entrance. You can kill them with the machine gun if you like. Approach it and press Interact to grab on. Move the mouse or stick to aim, and fire as usual. Press Interact again to let go.
Or, just shoot them with your own weapons, moving back and forth to avoid their gunfire.
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