-( Story related, cannot be missed )-
Being pursued by Ocelot after fleeing Groznyj Grad, you'll eventually come to face off against The Sorrow, or what's left of him, in a dream-like version of Ponizovje. This is a great reminder for a Peace Walker run, as any enemies you've killed other than the bosses will appear throughout the battle to sap your life essence. There are two ways to solve this fight, though one may not be recommended.
First way is the fastest: simply drown yourself. Drop down to crouching and climb through the water some until you're on your belly. Once you're there, just drain your stamina until it takes your life, then bam, you're dead.
The second way is the best for two reasons. This involves wading through the water and reaching the end, where the Sorrow is hovering over a corpse. This is suggested for the Peace Walker trophy as it shows you if there are any enemies other than the COBRA Unit that appear (which, if there are, means you've killed someone and screwed your run up for this trophy). The second is because completing it this way will end up unlocking a Camouflage option for you after the fight.
Either way, once you die and get the "Snake is Dead" screen, this is a false death similar to the fake death pill. For this reason, if you hold
, highlight the Revival Tooth and use it with
, you will return to the game and to life.
Note: Since you won't have your Survival Gear until you end up meeting with Eva behind the waterfall, you won't directly notice the camo being unlocked. Once you get your options back, go into the Camouflage menu and you will see the Spirit camo unlocked.