Start game as usual. Then use the following codes for 
accessing levels in EASYMODE gameplay:
 
Episode I 
---------
Level 05 : FORBABYS 
Level 10 : TERMINAL 
Level 15 : SHOOTERS 

Episode II 
----------
Level 05 : GOODLUCK 
Level 10 : BREAKOUT 
Level 15 : MEFODIUS 

Episode III 
-----------
Level 05 : TRYTHISS 
Level 10 : PREDATOR 
Level 15 : PREMIER