Really? I thought The Flying Machine was the easiest stage in Specter Knight's campaign, it was the first stage I completed in both my first and NG+ playthroughs. The new Propeller Knight fight I also found easy because the low gravity makes it easy to keep out of his attack range, and gives you plenty of opportunity to go on the offensive with minimal damage risk to yourself.

If you are having difficulty clearing a stage as Specter Knight, I suggest you practice the Horace tower climbing mini-game without using curios. Despite it being classified as a mini-game, its actually secretly a tutorial to learn more of Specter Knight's advanced platforming techniques.

It was easy after the4th chekpoint