If you fancy challenging yourself then complete the game on 'Casual' mode once and this new difficulty setting will which will test your skill in the game will become an available option.

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.