I'm in love with the art style. You did a really great job of evoking the printed page (the half-tone screens as shading really sell the aesthetic!). There are also lots of little pieces of flair that might go unnoticed (the fact that your kaiju announces its stomp move with a different name each time, for example) that really work well together. And all of the kaijus are adorable.
I played through several rounds and I also found the progression system to feel like it was tuned really nicely. I got obliterated on some of the early waves (maybe because I didn't yet fully understand what the spin thing was all about), but then seeing my kaiju grow up and survive more waves was really satisfying.
The sound design is also great. The music is a perfect fit for the theme, and I love the way it shifts into battle mode when you send a new wave.
All-in-all, there's a fun core loop here, and a lot of polish for a one-week project. Fantastic work!