Hi, I wanna try joining the challenge on this game, and I saw this as one of the rule:

"Complete all 10 challenges in the “Mettle of the Monkey” page in any order, achieving ACE scores on each."

What is an ACE score?

if you hover over the specific challenge you can see both the regular and the ace score requirements

I'll try it, if I still unable to find it then I might reply again here. But for now, thanks!

It's just reaching a certain amount of score in each ecific level, which is higher than the regular requirement. For example, in level 1 challenge mode you have to get 40k points for just beating it and 60k to get ACE score.