At first you will see humans abducting your clan mates. Follow the only way until you come to a grove. Yep, you've seen such locations a lot on the previous level and this one is the same. There are three guards down below. When you've dealt with them, jump down and head left. The road right is a dead end.
You will come to another grove. Two guards will take position behind the crates, but they are easy to kill. The next part is not really hard, but tricky. You need to go deeper into the human base. However, they have prepared turrets which will spot you and kill you if you're not too careful. Don't rush in and plan your step carefully.
Note: Turrets, synthets and aliens can see through your cloak.
At first you will see about four guards. Kill them all. Most likely, two more guards will appear. Don't go up any slope. See the turret up ahead? If you don't, try using night vision mode and zoom in. The turret will instantly kill you once you come closer. There is only one safe slope and it's the rightmost one. Go up and you will see a gateway and a single guard. If the guards haven't been alerted before, there will be more. If you walk away from the gateway, you will see three turrets total. They cannot attack you if you approach them from behind and thus they can be easily killed.
You can't go through the gateway, so go up the tree near it and jump to the cliff nearby. Most likely, a guard will appear, fire a short burst and run away to alert his friends. Follow the tunnel, kill the guard you've seen before and jump down. There will be three pairs of guards searching for you. Nothing special.
Now you need to climb up. Jump on a big tree, reach the cliff from it and follow the tunnel. It will lead you to an opening. Walk left and you will see a dropship on the left. Follow the tunnel until you come to another grove. A guard dead ahead will warn his mates. He's easy to decapitate, but once you jump down, you will encounter more enemies including a few dangerous ones.
There are five regular guards below, one synthet and a guard on a tree. Beware of a synthet since he (it?) can see through your disguise and has a powerful minigun. Luckily, there are lots of trees to hide behind, so once you become outnumbered, run away and take cover. There is a vehicle with a gun, but it's just a decoration. Your enemies won't use it no matter what you do.
Look for the highest tree in the grove. You need to reach the top of it, but its lowest branches are too high. You must reach the highest tree from the neighboring one. Save often in case you fall down and have to start from scratch. Once you reach the cliff, go through the tunnel to end the level.