There is a boss on the left side that opens the 3rd door, to kill him you must grab a box and throw it at him until you kill him. I hope to help you
Yeah, no wa in hell I can hit that door.
The best I can manage (by standing beneath the rocket shooter, one milimeter to the right and jumping)
is to get the rocket to fly super close past the grabbling arm block on the right, jsut closely flying around it without touching it.
and flying into the bottom platform, exploding there without any effect.
simply expressed:
the minimal movement radius of the rocket is jsut that much bigger in the actual game than in the video.no way that the rocket can fly past that middle block on the left side.
will try again, with the door now gone.