How to translate trial, challenge, struggle
For a general conflict, struggle, trial, challenge, etc., use balac (opportunity), because a Mando sees these sorts of situations as opportunities to keep from stagnating or as a means to improve.
A test of or challenge regarding a person's knowledge or ability could be considered balac par parjai, an opportunity for victory.
If you want to be specific, use a relevant adjective. One popular route is to add urakto or umaan, which are both listed as "difficult." One suggestion on the difference is that umaan relates to difficulties that are particularly dangerous or risky (because we also see maan in ramaanar, to die, and ramaanla, mortal/fallible), leaving urakto for use with other kinds of difficulties, such as those requiring effort.