Just as in Mission #31, the ideal equipment configuration for this battle is to use a rocket launcher to attack Sahelanthropus, an automatic weapon to destroy homing projectiles and fire at the floating boy, and night vision goggles to navigate the red mist. As this is a battle where fine margins can matter, taking your most powerful grenade launcher as your second primary and an accurate SMG as your main secondary will improve your options.
Finding the correct balance between aggression and caution is the key to winning this fight. In most other areas of MGSV, prudence is preferable: you might not get to your goal quickly, but you'll definitely arrive. Here, though, regular moments of bold and brazen offense are absolutely pivotal. If you constantly flee, waiting for clear opportunities to turn and fire, you'll almost never find them. Instead, you need to learn how to read Sahelanthropus, exploiting momentary post-attack pauses to fire several missiles in succession.
The opening seconds of the fight are a perfect lesson. You have time to fire twice with a rocket launcher before you are obliged to run to avoid a rock spire attack. After you dodge this, though, it's possible to unleash a few more rockets before you need to turn and sprint again. This, in essence, is the best way to fight.
If you can handle it, rotating the camera to view Sahelanthropus while you run away from the Metal Gear's attacks makes a massive difference. It's by doing this that you notice when he is briefly inactive, when he is distracted by a support chopper, or can read signs to intuit his next move. It helps to have a familiar rat-run, where you are completely comfortable navigating a loop within the hot zone without actually looking where you are going for much of the time. The rock-strewn slope behind your starting position and the road that links to either end of it is a perfectly viable circuit.
While the core stomach weak point is always your primary target, don't forget that destroying other body parts can be beneficial. Damaging the arms (to increase the charge time of the saber attack), the thighs (to cause heavy damage) and the white tanks on his back (to cause heavy damage) are all worthwhile gambits that may make a big difference.
Last, but by no means least, a simple tip that some players may forget: you must bring Big Boss to a halt for him to replenish his health. If you are perpetually sprinting to avoid Sahelanthropus, he simply will not heal. Wait for the Metal Gear to perform an attack that is usually followed by a pause - such as the rock spire attack - then duck behind cover until the red veil dissipates.
