This being the hardest mission, feel free to pack your combat vest with anything and everything you need. Grenades will be a great asset, as well a powerful pistol and assault rifle. Bring along a SMG for extra firepower at close range, and your taser.
When the mission starts, Lt. Vladik Savin will enter the area you begin in. Make sure he meets with a fatal 'accident'. Don't underestimate him - he's a heavily armed officer with a machine gun and is well protected as well. In fact, feel free to use grenades and headshots to drop him. Should he see you, he'll start sprinting, making him a lethal enemy. There's a box with Sarin grenades and a flak jacket inside the bunker here.
When Savin has fallen, enter the doorway he came out from. Kill whoever appears and prepare your grenades or scoped weapon. When you kick open the door at the top of the stairs, Lt. Rodion Ushakov will use his tank's cannon to blast you away. You only have a few seconds to kill him with a well-placed grenade or headshot.
Ignore the enemies here, and instead head east to the giant doors. These lead to the helipad where Stone is busy fending off troops. Before you do anything else, climb up to the copter and sabotage its controls. This will help you immensely, especially if you do not have grenades.
After you're done, access the hangar and grab whatever you need from the boxes. Stone will ask for your help rescuing Alima, but he really won't need it. Rather, shoot off the gate lock and descend.
When you hit the second floor, a heavily armoured soldier will be standing in front. Sift his skull and ride the elevator back up, but roll off so you stay here. Next, another troop will arrive. Waste him and then head towards the junction where Lt. Yegor Leonov spawns. You have 2 methods here.
- If you have Sarin grenades, kill his guards and let him escape. He will rush for the cargo elevator, so kill anybody who gets in your way and retrace your steps to the elevator. As soon as he shows up (use your radar), start tossing nerve grenades towards the large lift. When the doors open, he and his entourage will suffocate in a rather thick cloud of death.
- If you don't have Sarin, recall the elevator from the first floor. Leonov will head for his copter, but you disabled it earlier. You have 1 minute to return to the helipad, and if you succeed, he'll panic while trying to fly off - crashing into the hill in the process.
After dealing with the third officer, return to the second floor and send the elevator upstairs again, but make sure you stay here. This is essential if you die and respawn on the surface.
Using your radar, head for the last lieutenant - Oleg Petrenko. You may run into a good number of guards here, so kill them and grab whatever armour you can find. Eventually, you will be able to use an elevator to ascend to the surface. Slowly make your way up the stairs, and aim a well-placed grenade at the sniping lieutenant. Should you wish, a headshot or taser burn may also be a good substitute if you can kill him without exposing yourself.
With that, you're done on the surface. Head back to the elevator and go to the area where Lt. Leonov used the lift. Call this lift and go down to level 3. There are 3 methods you can use.
- If you have frag grenades, toss one at the M-79 guard. Just as you throw the grenade, he'll open up, and you'll both be killed.
- If you don't have frag grenades, or want to gun your way in, use the dual CZ machine pistols and the MDS-7. As the elevator descends, you have enough time to drop the M-79 patrol with a headshot with a very fast and stable weapon. You could also use body shots with the Barrett 50-cal or Desert Express. Even if you miss his head, keep firing - the man will hesitate a bit, giving you one more precious second, and you may just kill him.
- If you prefer to hide and run, roll before the elevator hits the bottom and get to the crates close to the lift controls, before the M-79 fires. Stay there and eventually, both guards will slowly approach you. Hopefully, this will give you a chance to nail them before you yourself get owned.
After killing the guards, head for the vent and follow it to the Omega Strain computer. Get the DNA code, and return to the elevator. Ride it to level 2.
Once there, return to the Leonov junction and follow your map to Gabe Logan himself. Watch out for enemies along the way. Let Gabe draw the enemy fire (he's invincible), and just kill the stragglers. Eventually, you'll have to protect him when he disarms the missile. Keep at it, and you'll have completed the last mission (and, by extension, the game)!