What trips up people the most is that they don't realize they need to climb their way back up and go through the door at the top right near where they started.
Just make sure you flipped the two switches on the right side of the map (Shown in green) and make sure you have a pathway back up.

Lv 50 is brutal. You start off at the top left-most side of the map, and have to zig-zag your way down.
Going straight down is not an option due to the indestructible blocks in the way.
Start by going down and to the right, then you have to go down and to the left. I only got as far as halfway down the map before dying from falling blocks, radiation, or running out of energy, so I don't know how much more zig-zagging you need to do to get to the end.
Did I mention you only have 6 minutes?
By the way the chest is at the top right of the map.