For easy, I'd throw in stuff to the tune of Reptites, Winged Apes, Kilwalas, Freelancers, etc. since otherwise it'd be reaaaaaaaaally easy. For the medium, I'd add the midrange hard stuff, such as Tubsters, Hexapod, Mohavor, Proto 4, Debuggest, etc.
Basically, I'm just thinking the entire coliseum should be on the higher end of things, where if a monster can be hard on a scale of 1-100, the coliseum would include those 33 and up. The hard part really is going to slaughter some people, which is what we want. However, it seems to be a pretty big jump from the middle right now.