So you want to make aÂ trulyÂ unique challenger in the arena. To figure out what you can play lets start with the basics.Â The basic CR rating for the battles is CR10. a 11th level creature without HD is a CD10 creature (class levels equal CR -1). So any CR10 creature can be used without modification.
You can also use monster templates on lower CR creatures to bring them up to CR10.
Another option is toÂ use a creature with HD then add class levels to bring them to CR10.
Finally if you feel really adventurous you can use a combination of HD creature, template and class levels to get to CR10.
Below is a list of creatures and attribute bonuses they get for level advancement to CR10.