Solid entry. I love the scribbly art style. I also like the wall mechanic where getting too close to the edge causes them to shoot you. That was super creative. The guns themselves are fun to use. I love how varied they are, and some of them come with their own disadvantages so they're all pretty balanced in that regard.
I think you should reduce the time it takes for the boss to spawn. The fight itself is very fun but it feels like he takes forever to spawn. The boss health bar should also be moved to directly above him. The first time I fought him, I didn't even notice the bar until he was almost dead. Finally, I don't think the gun that shoots exploding bullets should push you back. It's the only gun that affects your movement which makes it kind of awkward to use.
Overall, very fun and addicting game. I got a score of like 70000 before dying.