Anybody know where Concept Art 20 is?

[Gamechamp]

I'd been assuming it was just one per level and it'd be easy to find which I was missing, that doesn't seem to be the case. :X I know it must be in one of the final levels, but does anybody know which one? They're hard as heck so I really don't want to spend hours getting myself killed over and over looking for the chests when it may not even be there.