Best Leveling Location - Castlevania: Curse of Darkness
The best place for you to farm EXP for you or your Innocent Devil is in the Infinite Corridor. Specifically on the 2F right next to a Save Room you'll find a room with a large group of Skeletons that respawns every time you leave the room. In order for you to reach this area in the Infinite Corridor you'll need a Battle ID that has Brute Strength Level 2.
A full clear of the room in question will earn you about 9k EXP in total. Since a Save Room is right next door you'll be able to fully heal yourself and your Innocent Devil with each rotation as well. For the exact location I am talking about click on my map screenshot below. I have a little white line pointing to the correct room.
Depending on what Innocent Devil you decide to use here you can earn about 9k EXP per minute. With a Mage ID using a big explosion skill like Meteo, Demonic Disaster or Twinkle Star will wipe out all of the Skeletons instantly allowing the next group to spawn. After 5 or so groups of Skeletons the room will be cleared and you'll have to run back into the Save Room to reset them.
With any other Innocent Devil it'll be slower but it's still the best EXP in the game. Something I would recommend too is AFK farming while you're here - if that is your thing. Once you get to level 55 - 65 all of the enemies here will be hitting you for 1 damage each and you have probably over 500 HP. You can easily leave your character unattended while you do something else and come back every few minutes to reset the room.
Leveling up my Corpsey took about 2 minutes to clear the room and leveling up my The End (Devil ID) was about 3 - 4 minutes per clear. Each I did without playing the game and leaving them to farm all of the enemies for me. All in all I would say that you can probably hit level 99 at this location within a couple of hours. Innocent Devils will level faster especially if you're wearing the Brooch that gives them all the EXP.
If you'd like to see a video of me farming this location and also talking about it check out my Best EXP Farming Location (9k EXP per minute) Youtube Video.