Hard Boiled difficulty:
To unlock Hard Boiled difficulty, complete the game on Casual difficulty.
Secret theater + items:
On the first level right after the second checkpoint, walk in the door and take a right to find a damaged door. Shoot it and walk through the corridors. It will take you to a secret theater that plays previews of two games, The Wheelman and Blacksite Area 51. There is also a golden pistol, a paper crane, and a shotgun in the theater.
To get more ammunition for the handgun, use the Tequila Bomb for the Precision Aim, and shoot someone.
Wong and Dapang strategy:
To defeat Wong and Dapang, when DaPang jumps down to you, run straight ahead and shoot the bottom right of the dragon statue. Usually the statue will fall, killing Dapang instantly. You now have to be fast at this stage, especially on the higher levels. As soon as DaPang is dead, you will have a couple seconds to pick up the two paper cranes which fell after you destroyed the statue. Now at least five or six men will appear and run directly at you. Evade them as best you can and make your way to the golden guns to the left of the statue podium. Trade in your weaker guns. You will now have 32 golden bullets, as well as most likely a shotgun. If you do not, grab one. Make your way to the far left or right, then take cover behind a pillar and wait for the remaining enemies to come after you. When they get within a few feet of you, press Left or Right, Tequila time will start. Kill them with one shot. Usually there is one remaining enemy that shoots at you from a distance. Ignore him. If you kill him, more enemies will appear. By now you should have or almost have a barrage bomb. Run upstairs and pick up a health pack if needed. As soon as you reach the top Wong will start to shoot, so you must be quick. Turn on Precision Aim. It is far stronger now that you have golden guns. Wong is weak against Precision Aim. Make sure you hit his head for maximum damage. While he is stunned, grab the paper cranes to the far left and right. Wong recovers fast. Use Precison Aim every time you get a paper crane. There are two more paper cranes on the chandeliers. Swing on them to get them, but only do so after successfully hitting Wong. After you have hit Wong as much as you can with Precision Aim, he will still be alive but with minimal health. Turn on Tequila time and fire your golden guns. He should die within seconds. If you are skilled at throwing grenades, use them. However, they are slow and will leave you vulnerable.