Loop and Merge to iterate backwards on question fields

Hello!
I am designing a survey where I want the participants to select all the challenges they solved in an educational event. This survey question looks exactly like the event's website so that the participants can easily remember which challenges they had solved. The challenge difficulty increases with the number, eg: challenge 2 is harder than challenge 1, and so on.
After this, I want to drill down on a few of the highest-score challenges they selected. Essentially if they selected 1, 4, and 5, I want to ask more questions on 5 and then 4. I was thinking of using loop and merge, but it only iterates on question fields from 1-5, whereas I want it to go backward on the selected responses ie 5-1. I am thinking maybe this would need some custom coding, but I am not sure how to go about it.
Any help here is appreciated!