This challenge is basically an easier version of the Extreme Boss Rush challenge, with less monsters and easier fights. However, it gets rather tough at the very end- even STL and Koloth got beat initially by the later battles! However, it IS required, so you may want to train to lv. 80 in the Cave of Trials before taking on the challenge. As usual, if you're cool, you'll keep the final battle in this challenge a secret.
I just tried the challenge colleseum, I like the oppenents for the last battle by the way.