I was hoping that someone can tell me where to find some hidden challengers?

Thank you.

[Discontinued] Complete List of Unlockable Challengers

There is a list of all of the hidden opponents and how to unlock them. :)

i remember i unlocked one while browsing the site, many moons back [=last year in summer]. but i forgot how it happened, or which one that was.

however, many of them are unlockable through items available either in the cash shop or in the millionaire center. One such item is the Bloody Summoning Altar...

a few of them can be obtained from games. hope this helps a bit?

Thank you!