Level 3: Fort
Get to the fort ruins on top of the mountain
To begin with, just drive your inflatable boat onto the beach directly in front of where you begin the level and get out. Theres a bit of a hill thatll give you cover from the mercs on the dock nearby; there are four of them in total. Before the gunplay starts, use your binoculars to scout the area, and try to zoom in on the beach far in the distance; you should be able to tag a whole mess of the islands inhabitants and get them on your radar. Thisll help ensure that youre not shot in the back later on.
At any rate, when youre ready to begin the carnage, use your M4 to shoot the propane tank near the fishermen; thisll kill them both right off, leaving you with two more targets to dispatch. A few more of the soldiers will be approaching you from the south, so wait for them to approach and then take them out.
After things quiet down, youll be able to scout out the area a bit more thoroughly. If you stand near the building with the shark outside of it and use your binoculars to scout up and down the mountain to the south, you should be able to spot around two dozen soldiers combined, with most of them arrayed on the beach. A frontal assault is certainly possible, but on the harder difficulty levels, youll want to be a bit more subtle, so push your inflatable boat back into the water and head out to the east. Theres a small bit of land that juts out between the soldiers beach and another, smaller beach, where four more soldiers are mulling about. Land your boat here and walk towards the four soldiers, killing them as you approach. If you cant kill them all quickly enough, a couple of them may drive off in the buggy here, but dont worry; youll catch up with them soon enough.
From the beach, walk up the path into the jungle. Theres a set of well-guarded stone steps nearby that youll need to surmount if you wish to reach the ruins, so kill off the guards and take their weapons. If youre low on ammo, health or armor, you can follow the northern part of the trail and use the minigun above the beach to wipe out the soldiers there. Theres a set of supplies in one of the huts here, along with a very handy sniper rifle. You wont have a very good angle on the soldiers in the fort from the beach, so save your ammo for later.
As you walk up the stone steps, your path will branch again. The left trail leads into a bunker, while the right is a more straightforward path to the ruins above. The bunker is a bit longer, but it winds up being the safer of the two due to the ease of using grenades to kill the soldiers inside. Either route will get you where youre going, however. The bunkers main room also possesses a silenced MP5 sub-machinegun, which is a useful enough tool to justify the trip, in our opinion.
Whichever path you take will eventually lead you to the ruins, though. Work your way through it, methodically killing off the half-dozen or so inhabitants. On the forts north side, theres a small pillbox with a ladder leading to the roof; you can use this as a sniper spot, or to lure soldiers into the pillbox itself and grenade them to death. Whatever your methods, youll want to kill off the soldiers before proceeding towards the door to the communications facility. As luck would have it, its locked, but at least someone had the good manners to leave you a rocket launcher nearby. You dont need it to blast the door open - thats Doyles job - but you may want to keep it to deal with one of the incoming helicopters.
Defend yourself until Doyle hacks the door code
As Doyle hacks into the doors controls, youll come under assault by the aforementioned helicopters. Depending on where you move about, you may face a single helicopter of the guy-leaning-out-with-a-minigun variety, or a more advanced model thatll hover about while more soldiers rappel from its interior down into the fort. Either way, youll want to take cover, as theres also a patrol boat in the harbor below thatll start sending up mortar rounds near the entrance to the door.
The obvious course of action here is to just hide until Doyle radios you and lets you know that the doors been unlocked. If need be, use the rocket launcher on the minigun helicopter to encourage it to leave you alone, then time your dash towards the door so that you dont get blown away by the mortar rounds.
Get inside the comm center and kill the commander
The commander of the facility is in the door to the immediate left of the entrance. Hes a tough bastard, and his helmet prevents easy headshot kills, so either use a spare rocket round on him, or throw down a flashbang and get up close before finishing him off. Hes not incredibly more resilient than most other soldiers, though, so if youre out of these items, just whip out your M4 and unload a clip on him until he falls.
Find an explosive pack and plant near generator
The explosive pack you seek is in an armory near the commander; grab the keycard off of his body and use it to unlock the gate to the armory, wherein youll find the explosives and more ammo for most of your weapons.
The generator is down the hall from the control room, with a big red spot on it where youre supposed to place the explosives, so place away and start running like hell.