Here is the full list of all 100 Major Mayhem achievements.
Defeat 2,000 enemies with Grenades
Remove 10,000 enemy hats
Defeat 2,000 enemies with the Automatic Shotgun
Defeat 2,000 enemies with the Shotgun
Defeat 2,000 enemies with the Quadzooka
Play 100 Timebomb games
Defeat 2,000 enemies with the Minigun
Defeat 2,000 enemies with the Plasma Rifle
Deflect 1,000 projectiles
Defeat 2,000 enemies with the Magnum
Defeat 2,000 enemies with the Grenade Launcher
Defeat 2,000 enemies with the Automatic Sniper Rifle
Score 100,000 points in Timebomb mode
Spend 100,000 coins in the Armory
Defeat 2,000 enemies with the P99
Defeat 2,000 enemies with a Chicken Cannon
Defeat 2,000 enemies with a silenced weapon
Play 25 arcade games
Defeat 2,000 enemies with the Uzi
Defeat 500 enemies with a Chicken Cannon
Shoot 1,000 chickens
Have 10,000 coins in the bank
Defeat 500 enemies with the Grenade Launcher
Rescue 1,000 hostages
Defeat 2,000 enemies with the Pistol
Defeat 500 enemies with the Magnum
Defeat 500 enemies with the Quadzooka
Discover all the multiplier call outs
Shoot 4 chickens simultaneously
Complete all the 150 mission objectives
Defeat 2,000 enemies with the Auto Launcher
Use the Chicken Cannon and wear the chicken hat as the General
Unlock all the weapons in the Armory
Defeat 500 enemies with the Plasma Rifle
Defeat 100 enemy tanks
Defeat 500 enemies with the Minigun
Score 50,000 points in Timebomb mode
Score 20,000 points in Timebomb mode
Earn 3 medals on all Tropic missions in Classic mode
Defeat 500 enemies with the Automatic Sniper Rifle
Defeat 2,000 enemies with the Light Machine Gun
Defeat 1,000 enemies shirtless
Defeat 2,000 enemies with the Sniper Rifle
Defeat 500 enemies with the Automatic Shotgun
Use the Light Machine Gun and wear the red bandana as Rambro
Complete all the missions in a row without dying (Classic or Survival mode)
Completely dismantle the Giant Death Machine
Trigger 100 barrel chain reactions
Defeat 500 enemies with Grenades
Defeat 500 enemies with the Light Machine Gun
Shoot down 100 enemy jets
Finish the Desert in Survival mode
Earn a multiplier of x100
Use the Silenced Pistol while wearing the Tuxedo
Run 50 kilometers
Get 1,000 Close shaves
Defeat 2,000 enemies with the Bazooka
Defeat 10,000 enemies
Remove 1,000 hats from enemies that are already dead
Score 250,000 points in Arcade mode
Score 150,000 points in Arcade mode
Perform 100 dive rolls
Defeat 2,000 enemies with the Rifle
Shoot 30 chickens in a single Arcade game
Defeat all the red bonus ninjas
Defeat 500 enemies with the Rifle
Collect 10,000 coins
Get 100 double kills with the Shotgun
Defeat 500 enemies with the Shotgun
Rescue 100 hostages in a row
Finish Metropolis in Survival mode
Defeat 4 enemies simultaneously
Defeat 500 enemies with the Bazooka
Finish the Tropics in Survival mode
Defeat 500 enemies with the P99
Defeat 500 enemies with a silenced weapon
Defeat 500 enemies with the Pistol
Play Major Mayhem on 3 consecutive days
Score 50,000 points in Arcade mode
Play in all 3 locations during a single Arcade game
Defeat 100 enemies with barrel explosions
Use 5 or more weapons in one Timebomb game
Hit an enemy with every shot in a mission
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Defeat the end boss (Classic or Survival mode)
Play Major Mayhem for 2 hours
Rescue 5 hostages in a single Timebomb game
Defeat 100 enemies by shooting through an object
Purchase one of each of the 5 power ups
Defeat 500 enemies with the Sniper Rifle
Defeat 500 enemies with the Uzi
Defeat 1,000 enemies
Jump 20 times in one mission
Unlock all the game modes
Complete all the other achievements
Defeat 50,000 enemies
Earn 3 medals on all Metropolis missions in Classic mode
Earn 3 medals on all Desert missions in Classic mode
Unlock all the clothing items in the Armory