Well I understand the meaning of a challenge, and the whole thing should be catered to newcomers, but requiring people to level up 5 levels in a week is down right impossible in later levels.
What happened to me was that I was at 120lv when the challenge started, two catch up challenges passed before I could reach 125lv, and the new challenge started right after I reached 125lv.
So the only way I can actually do the challenge is to stop playing before I reach a multiplier of five, which should not be the developer’s intend.
What I suggest is to set it as a experience gather challenge rather than a level number based challenge, which makes it a lot more player friendly and achievable.