Once you free Jacob, you receive a shotgun. One well-aimed shot kills enemies. You only need to aim to the upper part of the chest and this should count as a headshot, for which you will be rewarded with thirty experience points. You will now take on a longer fight with opponents. The shotgun will work great here.
After you fight your way through all of the enemies, there appears a soldier with a flame thrower. He is a dangerous opponent, so if you are not fast enough, you will have to dash between the several covers in here or you will burn. To defeat the mercenary, you need to try to get around him quickly and shoot at the fuel tank on his back. You need three accurate shotgun shots to defeat him and still at this stage, it will prove useful.