Notice that the thwump will immediately fall as you begin the level. Wait there until the thwump almost reaches you, dodge to one side, and jump on top of the thwump (only the blue side is lethal). Ride it all the way up, and when you have almost reached the top, jump off to the left and get the doorswitch. Jump back to the column, get to the right side of the map (jump at the side of the center thwump to get it out of your way), and jump for the exit.