Discussion:DM Stumped by PC's Custom Classes.

So as a DM I decided that I wanted to let them make their dream characters. I wanted them to really get into their dudes and love them. Obviously, I couldn't just let them do whatever I want, but I wanted them to be as free-willed as possible. So one of the PC's created an extremely well balanced character class called the "Elder". The PC really knows what he is doing, and did a great job of the character. Another PC created the "Warden," which is a considerably powerful character that isn't quite as well balanced, nonetheless, I still want them to enjoy their characters. The third was the most unbalanced, called the "Guardian." I am stumped as to what I should do; because of the imbalance, the experience points calculator doesn't work the same, and they are able to take monsters with challenge ratings far above them, to the point of +10. I don't know if I should just throw really buff dudes at them and give them the experience, or make them dumb their guys down. I don't want to do the latter as much, cause I want them to really love their characters. What should I do?

Side note: This concerns the Guardian, who, through entirely unseeable and unpredictable events, wound up with skeletal arms hitched to him with a harness. The idea behind that particular encounter was to see wh