The Boss Extreme is the final and hardest difficulty in Metal Gear Solid 4. To unlock The Boss Extreme, the player must first beat Metal Gear Solid 4 on any difficulty.
On this difficulty, the player can only hold two rations, so it is advised to take caution when sneaking in the game as enemies can easily kill Old Snake in a matter of seconds. Guards and bosses deal considerably more damage per bullet and it would be wise to study certain techniques in order to defeat these bosses or to pass through acts undetected. Not only that but ammo is also reduced, for example the M4 carbine can only hold a max 300 rounds so use your weapons sparingly. The player can no longer buy anesthetic rounds from Drebin in The Boss Extreme, so stock up. They will however, start off with the maximum amount of anesthetic rounds. This does make a no-kill playthrough more troublesome if the player is trying to obtain the Big Boss FaceCamo.
Guards on this difficulty are extremely dangerous and are not to be taken lightly. Their senses are heightened therefore they can see you from a further distance and can hear more clearly. Anesthetic rounds don't last nearly as long so it is advised that you move quickly once you knock an enemy down. Bosses can be devastating without the right techniques or guns. It is advisable to fight them on easier difficulties and study their weaknesses and the environment.