After freeing the other 3 Spirits, return to the Ruins and to the Marle de Dragonne to make the Aqua Spirit appear. The darkness will be unveiled and God will be revealed to be the Demon Lord. You can now change party members. I personally prefer Melvin to Maribel. Also, you can go to Krage to fight a whole range of slimes, from normal slimes to king metal slimes. On top of that , all the monsters around there are ultra easy to beat and they will all add to your battle ratings so you can get class upgrades. The final Demon Lord isn't that tough if you got the right classes. 2 characters which can do Hustle and 2 fighters is good. Make sure they all have high HP though. The battle's still not a given. The Demon Lord has 4 forms.