More fellow metroidvania devs!
Ok so first and foremost...well done! This was super impressive for a month of work. I was already impressed..and then I found the boss, very nice touch. You also got saving in there! I just needed a feeeew more days to get that implemented into mine, alas.
I'm assuming, based on the "demo version - subject to change" that you guys are wanting to develop this into a full release? If that's the case, I hope you won't mind if I offer some constructive feedback based on my playthrough.
On my second playthrough, I accidentally found the end of the demo without ever encountering an enemy, though it happened fast and I can't remember how I pulled it off, but just something to watch out for.
The camera likes to jitter a bit, especially when the player walks up certain slopes. There were also some sections , when the player jumps, where the camera might follow the player vertically more than it should, made me feel disoriented while I was jumping (again, only sometimes.)
There almost feels like there's a slight input lag with the shooting. Barely, but enough so that sometimes when I missed an enemy it didn't always feel like it was my fault.
Lastly, I would use a much louder sound effect when the projectiles hit the enemies. You've got some great particle effects, and a nice juicy loud sound effect would make the projectiles feel nice and give them a nice feeling of impact. Currently, especially with the volume turned down I couldn't always tell if I hit an enemy or not.
I loved the hand-drawn art style of everything. The particle effects were great. The levels were interconnected, and I found it very easy to memorize my way around after a few loops (also a positive.) I like the dash quite a lot, very unique take with the effect! For a demo the difficulty is well balanced as well. Overall, very impressive work, I hope to see this on Steam someday!