Various Cheats:
While playing a game press ESC and quit, then go to Replay Level --> Bonus Game Modes --> then put in a code below

wawddssa - All weapons
wwswdaad - Unlimited Lives
dsdawsas - Unlimited Ammo
sdwwadsa - Invincibility

Get Gems for Extras:
After collecting the following number of perfect zone gems you get the following extras

35 - All Weapons
45 - Unlimited Ammo
50 - Extra Level
60 - Unlimited Lives
65 - Invincibility
78 - Everything

How to kill each mummy:
Slave Mummys:all weapons
Archery Mummys:shotgun
Swordsman Mummy:Guns
Spearman Mummy:Guns,shotgun,Lewis gun
Spirit Mummy:Amulets