You’ll face the Metal Gear Sahelanthropus in mission 31 of Metal Gear Solid V: The Phantom Pain. It’s not a very pleasant fight, so be sure to bring in your best grenade launcher and rocket launcher. You’ll also want to bring a fast-firing lethal pistol with you as it’ll come in handy towards the end of the fight.
Right at the start of the fight, fire your rockets at the Metal Gear’s torso, where all the white tubes are. You can fire at both the front and back of the torso. At the same time, Kaz and Ocelot will continuously call in the support chopper to provide backup fire. However, if the Sahelanthropus manages to destroy the chopper, you’ll have to manually call it in from the iDroid if you want the air support to continue. Kaz and Ocelot will also automatically call in supply drops each time you run out of ammo for a weapon, so if you don’t have a lot of GMP, expect it to go into the red during the course of the fight.
Aside from using you own weapons, you can also hop into one of the tanks lying around the area to do some damage. If you’re skilled enough with maneuvering a tank, you can easily shoot the Sahelanthropus while staying on the move.