There is only one way in

Save the damsel, but is she in distress?

Defeat 30 enemies in a row with your side arm

Defeat ten enemies in a row, alternating hand-to-hand combat and gunplay

Stand on the crocodile in the Secret Library

Inspect almost every display case in the Cartagena Museum

Finish the game in Easy 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 100 enemies with headshots

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

Defeat 30 enemies with the Arm Micro