The phrase "challenge level" refers to how difficult or hard something is. It is used to describe the level of difficulty or the amount of effort needed to accomplish a task or overcome an obstacle. Full definition
With more than 300 levels and regular free updates, these special episodes offer more challenging levels of pig - popping action and golden eggs to discover. (goodereader.com)
The most frustrating challenge level in the entire game. (gamingtrend.com)
These notes complement the specification and are intended to provide an accessible and information means of accessing some of the more challenging A Level topic areas. (tes.com)