The parkour was fun, but it could definitely be improved if you varied the parkour a bit more with blocks like ladders, fences, water, slime, ice, etc and incorporated it more into the build (a lot of it is just floating). It was also a bit easy; personally, I think it'd be better as a medium.
The build is nice, but "3d pixel art" maps like this aren't accepted anymore, unfortunately. It is definitely accurate, though.
The theme of a wither is original, but again, the "3d pixel art" thing is holding it back quite a bit.
Keep in mind, I am not an official map judge; take my review as that of a normal member of the community <3