Real fun game, although I'm pretty bad at it.
I'd say the requirements for the medals seem a bit odd; the ones for bosses seem a bit too strict for me, although attempting one usually means you might obtain the other too while improving at the game, which really isn't the case for the stages', as beating every enemy and doing a good time are pretty much mutually exclusive.
It also seems a bit odd to lock one of the player's ability behind beating Whispy Woods, even though I assume all of the Timed Challenge Medals requirements were set with it in mind ? Though it lets the player get familiar with the game first, which is a good idea.
An issue I encountered a few times though, which I suppose might just be due to the computer I'm using, is that the game freezes randomly, with it most often happening during the Dark Matter bossfight. Hopefully it's easily fixable and/or just machine dependant.