Codes listed below were made with the use of a
GameShark Pro 3.2, codes were hacked by myself, aka
Wanizame or my other alias SharkMe.

First the EXP codes. 
I'm only going to post 3 of them. 
1000 EXP After Battles - 
800E2064 03E8 
5000 EXP After Battles - 
800E2064 1388 
10,000 EXP After Battles - 
800E2064 2410 
Now you may Level up as much as you want and still be able to gain EXP. for classes. 
I toyed around with the GSCCC code for level 99. 
Hero at Lv20 - 30010C94 0014 
Gabo at Lv20 - 30010FF4 0014 
Maribel at Lv20 - 30010ED4 0014 
Melvin at Lv20 - 30011234 0014 
Aira at Lv20 - 30011114 0014 

Use those codes if you surpassed level 20 before finishing the Dharma quest. Go back to the past walk around battle to your heart's content, master a class go to the temple change a class, Dharma (past) is the best place to gain exp. for classes. 
Then you can re-level your characters by leveling up once (you'll level up from 10 to whatever level you were on, takes a while but hey you get "bonus" stats). 
I'd only suggest using 10,000 EXP after battles after you reach level 35, 5000 after reaching level 25, and 1000 after level 10. 
I also found out if you leave the Level 20 codes enabled, if you happen to level up you'll keep leveling up to Lv21 over and over until you turn off your gameshark. 
So I was thinking how to fix that. 
How do you keep from accidently leveling up? 
Here is how : 
Zero EXP After Battles - 
800E2064 0000 
Start with 500 HP codes : 
Hero - 80010CA4 01F4 
Keifer - 80010DC4 01F4 
Maribel - 80010EE4 01F4 
First use 'Tidy Items' 
Then use this code to get Inf. Items (mainly for herbs and Seeds) for the first 10 items. 

50000A04 0000  
80010426 0063 
Codes not hacked by me originator is unknown to me : 
No Random Battles - 800F265C 3500 
Quick Class Gain - 
D004B828 008A 
8004B82A A61F 
Notes - For the above code, you'll only need to be in 1 battle to master the class, its been found out in the GameFAQs DWVII forums that if you turn off the code, and fight 30 battles then switch to a certain class you can still mature to learn hybrid skills.