You can skip the whole second level. All you have to do is spawn the CPU player at the end of level one, despawn him by scrolling the screen and then shoot the top box after killing the boss.
It's a very specific setup - you have to destroy a certain number of items on the first level, then kill or despawn the CPU helper and finally after you kill the boss you need to shoot at the top box. The box must explode after the boss has died or it wouldn't work. The timing is crucial.
@@Kitto0 All weapons do the same damage per shot, with the exception of Iron's fire weapon. It does one hit per frame, so technically you can land multiple hits with one shot. You can see how it works in my 14:12 run.