I came up with it by fighting them, the ladder gets increasingly harder as you go up, starting on beginner with opponents who stand still for a large amount of time up to opponents who block constantly at stage 7.
However no matter where in the ladder you encounter these hidden characters their always the same difficulty its like their the same difficulty as shao kahn on expert (at least noob is, reptile seems to be a bit weaker).
Iv played against them on normal easy and beginner and no matter where in the ladder or what difficulty their always the same.