Kill the skeletons and hide behind the obstacles to prevent the enemy attacks. Pick up the health kit of you are injured and continue till you reach the portal.
You have no choice and no way back. You have to face these skeletons and kill them or be killed. Grab your gun and proceed further killing enemies one by one. Destroy the wooden boxes and pick up health kits or ammunition that will help you to move on. Shoot the barrels so they explode and destroy everything around.