This has the bones of an excellent platformer and just needs a couple of small tweaks.
Firstly, where's the win screen! It took be quite a few attempts to win and it was a bit anticlimactic to right back to the menu (unless it was a bug). The boss is pretty tough, near impossible to dodge or play around, but I had a run with a sniper and explosive bullet and they absolutely shredded him.
The audio & art is fantastic and there are a lot of animations here which I guess can go unnoticed sometimes, but I imagine that took an insane amount of work. I enjoyed the 'deckbuildery' bullet mechanic too, but I'm not sure I worked out what the cookie cutter strat was.
I had a great deal of fun with this, even if i nearly loaded the wrong game initially!