This was a fun little arcade game!
First off, the art looks great. The animations and particle effects do a lot here as well. I really like the bullet time mechanic. The game would probably be way too hard without it. I was able to make it through to the end during my first playthrough. I hadn't really been on top of my game for the entire time, so I only got a 'B' rating. I jumped back in to see what would happen if I just left the farmer exposed to the ninjas. Final score: 0 enemies, 0 crops, 0 score. Rating? Also 'B', so maybe another tier would be nice. Or I was just that bad my first time around, lol.