To fight Prinny Kurtis you need to find him in the item world and talk to him 5 times. You will find him in Innocent Town randomly. Innocent Town is randomly accessed after every 10 Stages in the item world. You shouldnt havent to grind this as you will be doing a lot of item world boosting in the post game but if you want to. You can use a Gency exit on every 10th floor to head back to the base. You should get an auto save at that point but its best to make a Save yourself in case you head back to the base by accident.
Simply Head to the exit and clear that Stage and you will either head to Innocent Town or be asked to go to Base or Next Stage. If you get Innocent Town, look for a Green Prinny and talk to him if he is there and continue on. If You dont see him or get prompted for the next stage. Quit to the title screen and reload the game and try again. If you are using the autosave DONOT select Return to Base as that will overwrite the autosave.
Once you speak to him for the 5th time, he will talk about you being a good opponent and want to fight you. You will then need to pass a bill to "Fight the defender of the Earth" through the Cam- Pain. I cant recall the amount of Mana.
Once passed you will have an option under the dimensional gate at the base after Episode 10 somewhere and you can fight Prinny Kurtis who is only LVL 150.