Sadly the game is bugged and unplayable, probably caused by ruffle.
Allthough the levels alone work, if entering a level by finishing the previous one, the map bugs and places walls, holes, crates, etc. at places where they shouldn't be. Each level HAS to be selected from the level select menu in order to work properly. Mostly.
However, I don't think that it's intended that all levels are selectable right from the start, I was able to play lvl 20 and 40 for the 2 boss medals without playing most other levels before them.
I was also able to enter the final boss stage, but that level is bugged entirely, even if selected from the level select screen. I get that there will drop 4 crates one after another which have to be pushed on the 4 buttons while evading all the holes and lasers. However, the second box that drops is ALWAYS bugged, can be pushed through walls and doesn't trigger any button. Since a 2nd button has to be triggered in order to enter the next boss phase and drop the 3rd box, the stage is broken and unplayable from that point on.
I tried restarting the level multiple times, both with R and from the level select screen, refreshing the page doesn't help either. I'm playing on chrome, but also tried in microsoft edge, still all the same issues.
I'm still giving a rather good score because apparently the game used to work properly back in the day, so the issue must be with ruffle or something relating flash player missing.
It's a shame I can't play the game how it should be, I'm sure it would've been a nice experience.