There are many requirements for unlocking the true last battle that don't have to be done in any particular order. First, make sure you complete all 6 Main Ops "Zadornov Search" missions, which are unlocked after the last mission (Peace Walker Battle 3). The Mother Base must also have been upgraded to 2 plants, which is achieved by first having a Level 40 Intel Team (just move a bunch of recruits to that section to increase its level) and then completing 13 Extra Ops missions (except Target Practice, No Limit or Training missions) to upgrade the base. It doesn't matter if the Extra Ops missions were already completed, you can repeat the same mission 13 times if you wish. You'll receive a message that the Mother Base has expanded. Do this twice. Once done, you must have collected all the Metal Gear ZEKE basic parts (collected by beating bosses) and must have the Railgun installed (which is gotten from S-Ranking the Chrysalis Battle). Also, you cannot have Metal Gear ZEKE assigned to an Outer Ops Team. After meeting all of those requirements, you'll trigger a call from Kaz informing you that Zadornov has escaped his cell yet again. So go to Extra Ops Target Practice #1, you'll see Zadornov on the 2nd floor. After the cutscene, the true last battle will be unlocked. If by any chance you don't get the call about Zadornov escaping that 7th time, then just keep completing Extra Ops missions until your Mother Base grows more and you'll eventually get the call.
ACHIEVEMENT - HOW TO UNLOCK Vic Boss (50 points) - Clear every achievement. MAIN OPS All Rank S (30 points) - Clear all MAIN OPS missions at Rank S. EXTRA OPS All Cleared (20 points) - Clear all EXTRA OPS missions. EXTRA OPS All Rank A (30 points) - Clear all EXTRA OPS missions at Rank A or higher. EXTRA OPS All Rank S (50 points) - Clear all EXTRA OPS missions at Rank S. Big Boss (50 points) - Clear all MAIN OPS and EXTRA OPS missions at Rank S. Fulton Recovery Veteran (10 points) - Reach 300 or more Fulton recoveries. Hold-Up Artist (10 points) - Reach at least 100 hold-ups. Lethal Enforcer (10 points) - Reach at least 100 kills. Stun Specialist (10 points) - Reach at least 100 stun attacks. Costume Collector (30 points) - Obtain every costume. Special Staff Recruiter (20 points) - Assemble all special staff members. Hideo Kojima (10 points) - Recruit Hideo Kojima. Mech Catcher (10 points) - Capture a mech. Paz's Thoughts (20 points) - Read all of Paz's diary. Huey's Thoughts (10 points) - Read Huey's love letter. Strangelove's Thoughts (10 points) - Read all of Strangelove's memoirs. Monster Hunter (20 points) - Clear all hunting quests. CQC Six Shot (20 points) - Perform CQC six times in a row. Leap of Faith (10 points) - Perform a Leap of Faith and obtain the Assassin's Straw Box. I feel... safe (10 points) - Put on a cardboard box. Indirect Attack (10 points) - Attack enemy soldiers by blowing up an oil drum or truck (single player only). OUTER OPS Sortie (10 points) - Carry out a mission in OUTER OPS. Elite Forces (30 points) - Clear "Capture the Enemy Base 06" in OUTER OPS using only soldiers. Trader (10 points) - Carry out a trade. Deliverer (20 points) - Exchange items back and forth with another player using Delivery. Paz Mania (20 points) - Obtain Rank S on a date with Paz. Kaz Mania (20 points) - Obtain Rank S on a date with Kaz. That looks tasty! (10 points) - Nicely cook a ration. VERSUS OPS All Rules (10 points) - Play using all of the rules in VERSUS OPS. Divine Wind (20 points) - Collect a soldier using a divine wind. An Army Without Borders (10 points) - Cleared Prologue - An Army Without Borders. A Country Without An Army (10 points) - Cleared Chapter 1 - A Country Without An Army. The Phantom Hero (10 points) - Cleared Chapter 2 - The Phantom Hero. A Nation Reborn (10 points) - Cleared Chapter 3 - A Nation Reborn. The Illusion of Peace (10 points) - Cleared Chapter 4 - The Illusion of Peace. Outer Heaven (10 points) - Cleared Chapter 5 - Outer Heaven. MAIN OPS All Rank A (20 points) - Cleared all MAIN OPS missions at Rank A or higher. Failed again, did you? (10 points) - Reached 50 or more game overs. There's nothing else you can do? (10 points) - Rolled 100 times or more in a single mission. Gives me infinite ammo (30 points) - Never worry about running out of bullets ever again. Stealth camouflage (30 points) - Never worry about being detected ever again. Kuwabara, Kuwabara (10 points) - Listened to radio near ghost. I'm dead serious (10 points) - Threw a chaff grenade at ZEKE (single player only). You're pretty good (20 points) - Shot enemy right in the middle of the rear end. Launched ZEKE (20 points) - Completed Metal Gear ZEKE. Completed ZEKE (50 points) - Customized ZEKE's head parts and foot parts with special AI weapon parts. Vic Vos (50 points) - Reached a total of 20,000 or more Camaraderie. Basilisk Eater (30 points) - Defeated Pupa, Chrysalis, Cocoon, Peace Walker. Steel Eater (50 points) - Defeated all AI weapons.