I found an "infinite MP" trick that could be used after chapter3!!
The key is Damien!!
He can transfer MP to others, including himself. This spell costs only 10 MP and can produce more than 10, depending on his INT, i guess. That is, if he is in the party, you don't need fish or trout anymore.
After switching party members, monsters will re-appear. Along with this infinte MP trick and a powerful mage(i love Dyluck, he's awesome) in party, it's very easy to level up at chapter4, because those monsters are very easy to beat and have high experience.
Now Dyluck is at level 35, kicking monsters' ass at chapter 5.