After a brief cutscene in which Lara enters the area and overhears Rourke on the radio, you're already in hiding amid the cane at the west end of the field. There are 10 mercenaries ahead, 3 fairly close to you, the rest off in the distance.
Move forward through the cane until you're close enough to the nearest man to see the Melee icon above his head.
Check to make sure he's unobserved (yellow in Survival Instinct).
Then press Melee to stab him and drag him into the cane.
Move forward through the cane a little farther as the second man comes to investigate. Then repeat the stealth takedown maneuver on him.
The third man should now have moved to talk to his buddy on the other side of the fence. Sneak in close to them but don't attack yet.
Wait until the buddy turns around and walks away, leaving the nearer man isolated. Now use the Melee attack...
...to stab and drag him just as you did the first two men.
Now there are 2 men ahead on the right side of the field and 5 more off to the left. Select Fear Arrows and shoot the guy standing out in the open.
While everyone's attention is focused on him, cross to the clump of cane on the left and move forward and left, past the rum barrels.
The man you just poisoned will probably only kill the guy on the far right (southeast) before expiring himself, but the mayhem should draw the 5 remaining men away from the left (northeast) corner.
You can then sneak directly to the exit, located inside the building at the east end of the field.
- OR -
If you want to kill the rest of the men so you can take their supplies and gold. Switch to Lure Arrows (by repeatedly pressing the Arrow Select button, 1 on the keyboard or D-pad Up).
Fire a Lure Arrow into the ground near where the soldiers are gathered.
As some of the men come to investigate and find themselves dying in agony, move back the way you came so they can't track the shot back to you.
Fire another Lure Arrow from a different angle.
Then move to another hiding place as the toxic cloud does its work.
After this, there should only be one or two enemies left. You can either take them on head-to-head or wait for them to move into the cane looking for you, sneak up, and finish them with a Melee attack.
Search all the bodies, gather resources, then enter the ruined building to the east. Before heading up the stairs into head up to the final combat area, use a dose of Focus and Endurance herbs and switch to the rifle. Stealth will not be an option in the next fight.