I've walked through this game myself. Honestly, I really like the idea that letting the ninja fight the robot, and there are many aspects that are deserved to learn.
Firstly, the art is really good. All sprites, UI icons, BGM, and sound effects match so well that gives me a strong feeling of being involved inside real ninja combat(I like the sound of the door opening and how it moves upwards). For the sprite, I like how the platform is not directly rectangle but with a sandstone outline underneath it, and the pixel unites well which makes me feel great.
Also, I like the mechanic of having both melee attacks and shooting, which fits the ninja setting a lot. Combined with the smooth player movement and fitted hitbox, the gameplay is fair and interesting.
The difficulty curves smoothly. I deal with a lot of level design from the previous assignment and exercise, and I found the more you test your own game the harder you can tell the level difficulties. But through my playing of your game, your difficulty for each level is designed very well and increases reasonably.
I think there are two obvious but super little imperfections that can be fixed further on like: 1. the sound of waving katana does not match with the animation but depends on how fast you click the mouse. It does not affect gameplay too much but might look and sound weird. 2. Glitch when the player goes through the platform. I think this should be a super common issue that also happens in our game project.
To sum up, it's a really nice game, and I really like it.