In the first level after the second checkout take a right when you have walked through the door. Then shoot the damaged door that you find there and walk through the corridors. This will take you to a secret theater that shows previews of The Wheelman and Blacksite: Area 51. You will also find in the theatre a shotgun, golden pistol and a paper crane.
Whenever you go into Barrage mode or Spin attack mode alway use a shotgun never your pistols or automatic weapons just a shotgun trust me it kills the guys faster and uses less time so you still have other attack mode use instead of losing it all because of your pistols or the automatic weapons.
Use Tequila Bomb for Precision Aim and shoot someone to create an extra 20-30 rounds of ammo when you have run out. Obviously to do this you need enough Tequila Bomb for the Precision Aim so try to keep enough in reserve for emergency ammo.