Treat every level like a real combat situation. Continually advance, but always get cover as soon as possible. Take as much time as needed, within reason. Never enter a building without throwing in a grenade first, or if there is windows, aim your sight at them from a safe distance while keeping cover. Your sight will show you where they are. Remember, if you do not advance quick enough VC and NVA will keep appearing in surrounding areas, including behind you in some levels. Discarding your M16 is always a must. Try using an AK47, as there is more than enough of them laying around. If done correctly, you will never be short on ammunition. Take the tunnels slow; sometimes you will get face to face with the enemy crawling the other way.