Successfully complete the indicated task to unlock the corresponding equipment:

Ambassador gunship: Successfully complete level 3 of the Helicopter Flying mission.
Diplomat heavy tank: Destroy the targeted buildings for the Allied Nations.
MOAB bomb: Successfully complete all HVT missions for the Allied Nations.

Win a level 3 weapon challenge (Grenade Launcher, Gun Emplacements, Pistol, and Sniper) bet at the PMC Villa with Fiona to unlock a random alternate costume. Each character has a total of five costumes. Note: Bet the maximum money for the level 1 and 2 challenges, then save and reload the game until you win. Bet the minimum money for the level 3 challenge.

Park your vehicle outside of a building you can enter, then enter the building and exit. Your vehicle should now be completely repaired with full ammunition.

Successfully complete all outpost missions before you initiate the final mission in Story mode. The outpost missions can no longer be accessed after the game is completed, which will also prevent a faction's shop items from being unlocked.

After completing the first UP mission and recruiting Ewan, talk to him about the Winch challenge. After completing level 3 on this challenge, bet the maximum amount of money. Even though you have already done this, the height does not raise. Therefore you can continue betting at the maximum amount. After approximately four times (depending on how much you start with), you should have about $7.0 million. Continue from here to get the maximum wager amount to $5.0 million (when you have about $14.0 million in funds). Repeat this as many times as desired. Additionally, when you get the maximum betting level on the Winch challenge, keep going until you get to approximately $100 million. The minimum bet will now also be $5 million. If you continue from there, the minimum will actually grow higher than the maximum and allow for even more money. However, once the minimum goes beyond $5 million, there is no way you can raise or lower the bet because it is higher than the maximum, but it is still the minimum.

Go to the courtyard behind PMC HQ (the villa from the first mission) to find several destructible objects that respawn, which you can use to easily get the "Hail and Kill", "Holy Smoke", and "Little Savage" achievements.