The easiest method to decapitate a zombie is, when you are equipped with the Shtogun or Assault Shotgun, let them get close to you, aim up and fire. Instant decapitation.
There is another method that works for both the Shotgun/Assault Shotgun and the Handgun. Press the auto-aim button (L if you're playing the Gamecube version, Z if you're playing the Wii version) and fire at an approaching zombie normally. You have about a 1/6 chance to decapitate the zombie with the Shotgun/Assault Shotgun and about a 1/10 with the Handgun. This tactic should only be used with the Shotgun/Assault Shotgun if you are fending off multiple zombies and can't risk letting the whole lot of them get too close for the usual method. This is highly useful with the Handgun, though.
Note that decapitated zombies will not return as crimson heads.
Herbs - all possible combinations and their effects
There are three types of healing herb found in the game - Green, Red and Blue. The below chart will list the the possible combinations and effects of each:
Green - Restores 25 percent of health Green + Green - Restores 50 percent of health Green + Green + Green - Fully restores health Red - No effect alone Green + Red - Fully restores health Blue - Cures poison Green + Blue - Restores 25 percent of health and cures poison Green + Green + Blue - Restores 50 percent of health and cures poison Green + Red + Blue - Fully restores health and cures poison
Note - No more than three herbs can be combined together. Therefore, it is not possible to combine three Green herbs and a Blue herb. Also, Blue herbs cannot combine with Red alone.
Note 2 - First Aid Sprays, the only other healing item found in the game, fully restores health.