Nice, had fun with this! The risk/reward calculation of getting close to bullets in order to reflect them back on enemies is good design, and killing the invaders is satisfying, making for an enjoyable core loop. My only major complaint was that the window limitation felt kind of tacked on.

Minor feedback (mostly just opinions, really) in no particular order:

• the cooldown on the swing is mildly annoying. imo, it would feel better if either you could swing as soon as the previous swing animation finishes, or if the cooldown was indicated somehow (maybe a meter, or a ding when the cooldown was done)
• it took me a while to figure out that bullets always travel in the direction of the mouse pointer. I think that’s because the swing animation looks kind of like a baseball swing, which made me think the reflection direction would depend on what part of the sword hit the bullet. making the swing animation faster might help with that
• unsure if this was a bug or a conscious design decision, but when reflecting bullets, they go faster the farther away from the character the pointer is. I’m not a huge fan of that - having your pointer close to the character already makes reflections less accurate, so it doesn’t need the extra nerf. suggest normalizing the direction vector for reflections and multiplying that by a constant speed
• the high score screen stayed up a lot longer than I wanted to look at it. would have preferred a restart button or a “press any key to continue” prompt over the timer