There are several requirements that have to be met before unlocking the true last battle. They 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). Next, the Mother Base must 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 two times. Next, 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 fulfilling 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.