Most of the challanges I've done so far was made by myself but I've done some that others come up with as well. Like level 1 party against Ozma in FF9.

However, my worst battles so far would be Omega and Ultima Weapon in FFVIII without junctioning (only allowed to junction abilities. Draw, magic, GF, item etc.) And with only the first weapons and no drawable GFs. Oh, and ofcourse, no card refining soo... no 100 heros or something. Took me alot of hours. :rolleyes2