The mechanics of the game are interesting and unique, but a bit confusing (as people have said regarding the functions of the weapons); the near-total randomization of the enemy waves and weapons also makes the gameplay feel like it relies a little too much on chance to be complete as a game (which could use a little more of a sense of progression).
Overall, it looks great, and sounds exactly how it should sound to fit the aesthetic - no complaints in either of those areas, although I only gave the looks 4 out of 5 stars because there's just not a whole lot of variety to evaluate.
The implementation of the theme was decent; there is obviously a light show involved, visually, and a couple of the weapons have light effects, but all of that kind of felt secondary to the point of the actual game, which feels more music-based than anything.
The limitation "Only One" is supposed to "be a technical or a design limitation", and I don't feel like the one life/one character aspects quite fulfill that limitation; many games only allow you to get hit once, but it sounds like the limitation is meant to apply more to things like assets, controls, or other aspects of the development itself or the way you have to navigate the game. Having one life isn't totally irrelevant, but it's not a unique implementation of the idea of there being "Only One" of something.
Overall though, I like that it looks and plays almost like a rhythm game, but not quite - it feels like some kind of twist on a shoot-em up with a rhythm game aesthetic. That combination may have been done before to some extent (as the whole thing kind of reminds me of some Japanese arcade games), but this implementation is definitely a unique experience. It's memorable enough that I'd like to see it expanded on and developed further into a more complex game with more progression. Given the limitations you guys were working with, you did a really good job; I really do want to see more!