The description for this achievement is misleading, as it will only pop after completing Mission 45,
This mission will start automatically after the completion of Side Op 150,
without giving you a chance to return to the ACC and change your loadout before thrusting you back into the action, so make sure you have a rocket launcher (I used the CGM due to its effectiveness against the helicopter) before starting the Side Op. (Note: If you're underprepared for the mission, you can't exfiltrate through normal means, but if you Fulton the materials crate next to the palace and ride the crate out, it'll force quit the mission and allow you to change your loadout before attempting it again) This is a perhaps the most frustrating mission in the game, but there are a few strategies that can be employed to make it more doable.
- With your CGM-25, take pot shots from various points of cover. The ones I used were the prisoner holding cells to the left of the palace, openings in the front of the palace, and peeking around the right corner of the palace.
- Change your location after every shot. This sounds tedious and unnecessary, but the enemy AI in this game is unfair in that it will lock onto your last seen location after every shot you take. Switch locations frequently if you don't want to be one-shot.
- Let Quiet handle the individual soldiers, focusing your fire on the heavy infantry. In particular, prioritize the missile array vehicles over the standard tanks, as those will frequently lay down barrages against you and Quiet.
- As far as ammo restocks, there are two options: If you wait a moment after fully depleting your ammo, Quiet will jump to your location and place a flare calling for more ammo, at no cost to you. This is the more GMP-efficient option, but results in long periods where you're helpless with an empty launcher. As such, I instead opted to call supply drops in well in advance, usually requesting a new one every time the previous one dropped. The ground behind the palace serves as an effective blind spot for the attackers, making this a good place for the supplies to be dropped.
It still took me a while, but these strategies helped me finish the mission and pop the achievement. I've been told that there are a few other strategies that can be employed, at the cost of your A-rank. If you get fed up with the mission and just want it to be over, or you're replaying the mission for the sake of the extra tasks, I'd recommend using these.
- In the right sub-menu of the iDroid, under the "Fire Support" option, you can call in a request to change the weather (this option unlocks after having a high enough Support Unit level, not sure of the exact level required but it seems to be somewhere in the 30's). By calling in a sandstorm, the visibility of the attackers will be severely impaired, allowing you to more easily Fulton the vehicles out.
- Additionally, the more prevalent means of completing the task of extracting all the vehicles is the equip a highly levelled Parasite Suit with the Armor ability, then popping the ability right before running up to a vehicle and Fultoning it. This method allows you to tank the incoming fire, but requires a lot of investment in developments beforehand.
I hope this helped, good luck!