What is 越级挑战 in one English vocabulary form and if possible, also in idiom?
You should provide the context in which this was used. It can be applied to school, work, development, sports teams moving up the leagues, anything.

越级: more than 1 level, skip a grade, leapfrog
挑战: challenge

越级挑战: some kind of test or exam or challenge. If your score is good, you go up more than one level. Could be school, work， just the children's game or whatever.

Not exactly your phrase, but you can see how 'leapfrog' might be used.

The win allowed them to leapfrog three teams to gain second place.

这场胜利使他们连超三个队，跃居第二位。