Bonus equipment - 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.
Nuclear Bunker Buster: Successfully complete the final contract mission, "Get Solano Part 2" to unlock it for purchase from Eva for $1 million.

Alternate costumes - Successfully complete a level 3 weapon challenge at the PMC Villa with Fiona to unlock the corresponding costume in order for the indicated character. There are a total of three costumes for each character. The Pistol challenge is the easiest to complete. The Emplacement Weapons and Grenade Launcher challenges are about even; however, Emplacement Weapons is slightly more difficult when you have to shoot the flying cars. The Sniper challenge is the most difficult to complete.

Jennifer Mui

Tactical
Sleeveless
Catsuit

Chris Jacobs

Camo
Off Duty
Chicken

Mattias Nilsson

Suit
Jacket
Chicken

Easy vehicle repair - 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.

Getting 100% completion - 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.

Easy money - 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. Note: To save time in the Winch challenge, use stackable items from outside of your stronghold. For example, try the gas canisters across the road (or for something less volatile, use the shipping containers directly across the lake behind your house). Items used from outside of your stronghold stay in the exact location you put them until you quit your game, giving you a prepared stack height the next time around. Two or three attempts may be required to gather the items, but it is well worth it, as once done it requires only seconds to win the challenge.

Easy challenges - 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.

Easy Sniper challenge - Use RB and not the scope for the Sniper challenge. Take out the drums next to the cars first, then go to the two statues for some extra time.

Easy transport and smuggling missions - Note: This trick only works after you have obtained a helicopter pilot and the winch ability. When on a transport or smuggling contract that relies on you not tipping the vehicle and losing the objects inside, summon a helicopter as close to the vehicle as possible before entering the mission assigned vehicle. Then, enter the mission vehicle, and the timer should start. Get back out, and take off in your helicopter. Use the winch to grab the mission vehicle, and gain as much altitude as possible. If you fly relatively level, you will not lose anything. However, if you bank even slightly to get around obstacles, losses will be minimal as long as you do not actually bank the copter in a hard turn. The vehicle will get tossed around, making the copter harder to fly; but if you travel directly to the drop-off point from the beginning, it will keep its cargo. This helps you avoid nearly all ground forces. All you have to worry about are the anti-air guns and other helicopters. If worst comes to worst, simply drop the vehicle after coming down to about 20 feet or less. You will have already have saved enough time to hijack another helicopter.

Stealing fuel without upsetting a faction - This is most useful when used with factions who are already hostile (to keep them from shooting you down), but can also be used to keep friendly factions from becoming upset with you. Get one of the helicopters used by the faction you want to steal from (or one of its allies), and allow your disguise to finish. Fly to the location of the fuel you want to get, and lower your winch. Grab the fuel tank, and lift off before the enemy soldiers recognize you, then fly to a secure location (either the PMC or a remote, open location). Once out of sight, call Ewan in for the pick-up. Note: Using a heavier helicopter is recommended, as the Rogue and Gunney class choppers are extremely difficult to control when carrying a heavy fuel tank. Also, there are at least half a dozen fuel tanks relatively close to the PMC, most of which are controlled by Universal Petroleum. Spending the fuel for an Ambassador Gunship (an Allied helicopter, who are allies with UP) is well worth it if you are going to use your PMC as a fuel depot.

Use the following trick to steal fuel from a friendly faction without having to worry about affecting their status towards you. Go to an outpost that has fuel tanks, and tag them. Then, call in Ewan for the extraction. If a soldier happens to call in a report about your hostile act, go inside the outpost to cancel the call. By doing this you will get fuel and the faction will still be friendly with you.

Sniper rifle scope on regular rifle - Have a sniper rifle in your inventory. While you are using the scope, press D-pad Right to switch weapons. You should have an assault rifle out while having the scoped view.

Defeating Carmona - When Carmona's men are attacking the PMC, after you destroy the tanks, set a charge of C4 on the gates closer to the mansion, and detonate it. It should drop straight down and block the road leading out of the compound. Enter the mansion, and trigger the intermission sequence. When you exit the building, the road leading out of the compound will still be blocked. Kill Carmona's bodyguards, and his jeep should be stuck in the rubble of the archway. Kill him, and snap the picture for easy verification of Carmona.

When completing the mission on the oil rig in the water behind the PMC, kill Blanco and take his weapon, the hunting pistol. The hunting pistol is a very powerful weapon, often killing enemies in one shot. With the hunting pistol in your possession, continue through the "Finding Solano" mission. After the encounter with Carmona inside the Villa, he will run in a straight line directly away from you, giving you a clear shot at his back. If you miss on the first shot, he has a good bit of a jog until he reaches his jeep, giving you plenty of time to gun him down.

Easy "Hail and Kill", "Holy Smoke", and "Little Savage" achievements - 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.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Tattooed Millionaire (25 points): Earn $1 million.
Partners in Crime (50 points): Play co-op with a friend.
Pipeline (20 points): Capture all outposts for a single faction.
Eat the Heat (10 points): Try accelerating and braking at the same time.
Armaggedon It (20 points): Destroy 20 buildings.
Shoot to Thrill (20 points): Get 25 headshots.
Damage, Inc. (50 points): Use 20 airstrikes in co-op.
Hail and Kill (20 points): Destroy 50 objects using melee.
Holy Smoke (20 points): Destroy 50 objects with a grenade.
Little Savage (20 points): Destroy 50 objects with an RPG launcher.
Balls to the Wall (20 points): Beat the Winching Challenge three times.
Gone Shootin' (20 points): Complete one of Fiona's challenges.
Heavy Metal Thunder (10 points): Successfully complete 10 action hijacks.
Be Quick or Be Dead (10 points): Snipe 50 enemy vehicle drivers.
No More Mr. Nice Guy (10 points): Make all factions hostile.
...And Justice For All (50 points): Verify all high value targets.
Digital Man (20 points): Unlock all shop items.
We'll Burn the Sky (50 points): Unlock all landing zones.
Highway to Hell (10 points): Complete three different races.
Wheels of Steel (50 points): Come in first during a co-op race.
Nothin' But a Good Time (50 points): Destroy 200 objects from the gunner seat in co-op.
Everybody Wants Some!! (50 points): Play with a Mercenaries 2 developer or a player who already has this achievement.
Billion Dollar Babies (25 points): Earn $1 billion.

Additionally, there are seventeen secret achievements:

Wild One (20 points): You recruited Ewan.
The Rage (20 points): You completed "Rescue Carmona".
Hello, Hooray (50 points): You acquired the PMC.
School's Out (20 points): You completed the tutorial.
Ride the Dragon (10 points): You hijacked the tank in the villa.
The Analog Kid (20 points): You recruited Misha.
Oil and Gasoline (20 points): You recruited Eva.
Runnin' with the Devil (20 points): You verified Blanco.
You Better Run (20 points): You verified Carmona.
No Compromise, No Regrets (20 points): You battled for Caracas.
The Hero and the Madman (50 points): You completed Mercenaries 2.
Dirty Deeds (10 points): You achieved maximum Universal Petroleum mood.
Island of Domination (10 points): You achieved maximum Pirate mood.
Forever Free (10 points): You achieved maximum P.L.A.V. mood.
Longing For Fire (10 points): You achieved maximum Chinese mood.
Stand Up and Shout (10 points): You achieved maximum mood with the Allies.
Aces High (50 points): You captured all high value targets.