General info about the mission
The last, fiftieth mission in Metal Gear Solid V is one of the repeated missions. The objectives are identical as in mission 31, which is the last in the first chapter. The description of the battle (types of Sahelanthropus's attacks, tactics of the boss, etc.) can be found in the walkthrough to this mission.
Defeating Sahelanthropus in Extreme mode is of course much more difficult. All his attacks are stronger and Metal Gear is very durable, which requires you to use better equipment. What's more, you can't use Reflex Mode (except for the finishing phase, when The Floating Boy appears) and Chicken Hat.
On the next page, you will find some tips on how to defeat Sahelanthropus in the Extreme version of the mission.