I did this for about 10 min , damn , that was hard
Here the way : when you get to the top with the level , you pull the level ,there will be 2 more platforms appear ,right ??? Now quickly jump to the lower platform ,and then jump to the higher one. When you jump to the higher one , stand in the centre , then a chest will appear down that the door. Right away you jump down and run to the chest. You have to act quick , because the 2 platform , and even the chest will disappear for a few second
Good lunk
Sorry for my bad English skill