Unockable: Mercenaries Mini-Game
Beat the game to unlock the mercenaries mini game.

Unlockable: Punisher Weapon
Shoot 10 of the fifteen blue medallions in the game. There is a map you can find in the farm area just beyond the first village with the bell tower. After collecting the map, the locations of all fifteen medallions will appear on your map. When you’ve destroyed at least 10, talk to the merchant for a free gun.

Unlockable: Assignment Ada Mini-game
Beat the game to unlock the mini game.

Unlockable: Broken Butterfly Weapon
In the hall south of the hedge maze there is a locked door. Return to that area with Ashley, piggy back with her and she will unlock the door. This reveals the broken butterfly.

Unlock Matilda Weapon
Unlock the Matilda handgun (which fires 3 round bursts) by clearing the game.

Unlockable: Chicago Typewriter Weapon
Beat Operation Ada, and the “Chicago Typewriter”, a Thompson with 10.0 firepower, infinite ammo, no reload, and the fire rate of a maxed out TMP, becomes available at the merchant.

Unlockable: Rocket Launcher
In order to unlock the Rocket Launcher Special you beat the game on Normal and you get a chance too buy the Matilda and the Infinite Launcher. The Matilda is $70,000 and the Infinite Launcher is $1000000.

Unlockable: S.T.A.R.S. Uniform
To Unlock The S.T.A.R.S. Uniform You Must Complete The Game On Normal Or Hard Mode.

Hint: Maximize Leons Lifebar
This process incurs several continues, which have no bearing on the game’s ending. At the first El Gigante fight, there is a Yellow Herb in the boss arena. Take this item and combine it with any of the Green Herbs in the same room and use it. This increases Leon’s lifebar slightly. Now let the boss kill Leon. Since the boss’ room is at a map transition (the green OPEN command is a dead giveaway), this lets you continue right at the boss fight but with Leon’s life increase from your previous attempt. Repeating this process will increase Leon’s lifebar to maximum prematurely. Extra Yellow Herbs found later can be given to Ashley, or sold to the merchant for money.

Hint: Snake in the Box
Sometimes when you destroy the wooden crates a snake will jump out and bite you. If you keep slashing with your knife when the box breaks you can kill the snake and receive a healing egg.

Hint: More Money for Beerstein and Elegant Mask
When you find these items do not sell them right away. You can increase the value of the items with gems.

Hint: Reload Your Upgraded Gun
Whenever you upgrade your gun’s capacity it fills the weapon with ammo. So don’t reload before you buy the upgrade. This works great with weapons that have very little ammo (such as the magnum revolver ‘Broken Butterfly‘); after you use up the gun’s magazine, simply look for the merchant and upgrade the ammo capacity.

Hint: Quick Aiming
There is a quick way to center your gun when aiming up or down. Simply tap the L1 button (takes out your knife) and you will quickly aim straight ahead.

Hint: Double Broken Butterfly Ammo
If you buy the broken butterfly before you find it, you can hold twice the ammo. The shopkeeper will not sell you this gun if you already have it.

Originally from IGN