Been caught up with work lately. However, the way I did this is; when you go up the hill and there are the enemy strikers/BRT w/e they are called. Go around them through the forest but go across the lake. Your screen should mysteriosly go blood red. Now run back into the forest and continue the mission without dying. When you get to the sub simply jump down in the hole. Even if you did enable unlimited health there is a death barrier right when you hit the floor of the sub. That's why its blurry