To complete a medallion you need to
- find the 3 hidden cages
- beat the electoon time trial
- Collect enough lums (300 most of the time, but in some level it's only 175)
You don't need to obtain the medal that require 350 lums, don't worry.
1) The electoon time trial is really easy to beat, especially if you are looking for the cheev "Nitro!"
2) To find the hidden cages and collect enough lums, it could be difficult for some level, and it's really boring to finish a level 3 times with 280 lums or something like that. So I suggest you to follow this walkthrough, it helps me a lot :
In these videos, the guy finishes all level with 300+ lums, and finds all the hidden cages.
I TAKE 0 CREDIT FOR THESE VIDEO, ALL CREDIT GO TO IGIPNP
THANKS TO HIM
Sorry if my english isn't perfect, I'm french
Good luck !