When you beat the game in Normal mode this difficulty setting which is a lot more tougher to complete will become an available opton.

When you meet the indicated requirement the corresponding piece of equipment will become unlocked.

Unlock Ore Additive:
Have 2 Guerrilla Actions.

Unlock Proximity Mines:
Have 5 Guerrilla Actions.

Unlock Personnel Detector:
Have 8 Guerrilla Actions.

Unlock Grinder:
Have 12 Guerrilla Actions.

Unlock Quantum Multiplier:
Have 18 Guerrilla Actions.

Unlock Stonebreaker Sledgehammer:
Have 25 Guerrilla Actions.

Unlock Shattersmasher Sledgehammer:
Have 35 Guerrilla Actions.

Unlock Level 1 Armour and Rocket Launcher:
Liberate Parker sector.

Unlock Level 2 Armour:
Liberate Badlands sector.

Unlock Nano Forge and Guerrilla Express:
Liberate Dust sector.

Unlock Thermobaric Rocket:
Liberate Free Fire Zone sector.

Unlock Jetpack:
Liberate Oasis sector.

The following 16 hidden challenges in Multiplayer can be completed in Matchmaking, Custom Matches, or a combination of both modes. 6 of the challenges must be achieved in one life while the others are overall (use the 'Statistics' view to see your position in the challenges and not the 'Leaderboards').

1 Life: 10 Kills - 1 Weapon:
Kill 10 people with the same weapon in one life.

1 Life: 15 Bonuses:
Get 15 different bonuses in one life.

1 Life: 25 Bonus XP:
Gain 25 bonus XP in one life.

1 Life: 5 XP - 5 Backpacks:
Get 5 XP with 5 different backpacks in one life.

1 Life: 50 XP:
Gain 50 XP in one life.

1 Life: Kill - 5 Weapons:
Kill 5 people with 5 different weapons in one life.

10 Kills - Every Weapon:
Kill 10 players with each individual weapon.

10 Wins - Every Game:
Type Win 10 matches in every game type.

100 Wins:
Win 100 matches (any mode).

100 XP - Every Backpack:
Gain 100 XP with each individual backpack.

100 XP - Every Weapon:
Gain 100 XP with each individual weapon.

1000 Kills - 1 Weapon:
Kill 1,000 players with one weapon.

5 XP - Every Bonus Event:
Gain 5 XP with each bonus event.

500 XP - 1 Bonus Event:
Gain 500 XP with one bonus event.

5000 XP - 1 Backpack:
Gain 5,000 XP with one backpack.

5,000 XP - 1 Weapon:
Gain 5,000 XP.

Select 'Options' at the 'Main' menu and go to 'Extras'. Then enter the following codes without the quotes to enable the corresponding feature.

Unlock Golden Sledgehammer (Single-Player):
Enter 'HARDHITTER'

Bonus Multiplayer Map Pack featuring 4 Maps:
Enter 'MAPMAYHEM'