You have just entered a quite vast location, occupied by a dozen-or-so opponents. You can take them all down silently. This requires a bit more attention, of you. Use bushes. You can hide in them and, as long as you are operating in shadows, guards should not spot you.
A bit ahead, it can be more difficult, because guards are more thick there and they patrol the area in twos. Use the advantage of height and kill the single guards from the wooden platform, or from the branch that you can climb onto over the trees stripped of bark. The key to success here is patience. Do not act hastily!
You can eliminate single opponents with bow and arrows, aimed to the head. Make sure that they do not get spotted, at the moment of dying.
After you defeat the initial few opponents, you can climb over the wooden wall. Climb over broadhead arrows that you shoot. Drive them into the bottom and the upper parts of the wall and jump up over them.
Watch out for the guards, for the whole time. If only opponents separate, try to kill them from behind, without raising the alarm.
You will have to swim ahead, a bit, but do not come out of water, after you resurface. Instead, swim up to the crouching opponent and press Y button to pull him into water. Climb onto the fragment of ice and deal with the other opponent a bit ahead.
There are only few opponents left. They can unleash lots of firepower, so try to remain in hiding for as long as possible. To eliminate double patrols that are unseen by the rest of the opponents, use bottles by the tent (this is where you came out of water). You can craft them into molotovs and throw in between the opponents. If you have plenty enough poison arrows, you can use them also.