There is only one way in

Save the damsel, but is she in distress?

Reach level 10 with one character.

Kill a monster in a single blow.

Finish the game in Normal Mode

Defeat 30 enemies with the T-Bolt Sniper

Defeat ten enemies using hand-to-hand combat on the back of the convoy trucks

Defeat 20 enemies by blind-firing while in cover (without aiming with L1/R1)

Defeat ten enemies with propane or acetylene tank explosions

Defeat 100 enemies with headshots

Defeat 20 enemies with gunfire by aiming while hanging

Find 20 treasures

Defeat five enemies in a row with headshots