Some codes have already been made (like inf. life, sp... Check codemasters-project or gamefaqs for them).
However here are some I created myself :
Code:
Press O is always allowed *
20224BF0 00000000
It means that during fights you can press O 5+ times in a row, and the character will attack five times without waiting (the game kinda buffers the button pressed). So you don't have to press O all the times, only here and there. Use Square if you want to cancel the attacks.
Code:
Enemies have 1 HP (=one-hit kill)
#1:
20243718 00000001
#2:
20243BA8 00000001
#3:
20244038 00000001
For the next ones, just add 0x490 to the address (I'm not sure what the max # of enemies can be).