Hot damn, this game is REALLY good. Five stars all around. And made in only 3 days? I'm realizing that the optimal game jam strat might be to make something really small and just polish it up to a tee. That's what you've done here, and it works.
The artstyle is great. It's what made me think "I have to check this out" when someone on my team linked it on Discord, and it absolutely delivers. The 2-bit color palette you're rocking looks super good, and the low-shading style works wonders here.
The sound effects are great (bfxr my beloved). Dealing hits, getting hit, buying things from the shop, all of these sound amazing. I especially love that juicy, succulent explosion sound (bfxr is great for those).
The gameplay is great. The flail is surprisingly tough to use, but not so much so that it feels out of the player's control. You have to get the right rotation speed to get the flail moving anywhere, and you can't move the cursor around too fast lest the flail snap back to you. It's a satisfying balance to pull off. There's also a good amount of enemy variety, which is appreciated. The quick guys in particular make the later waves really hard. I got stuck at wave 10 until I learned to discern when I needed to carefully move my mouse to dodge the 15 enemies on screen and when it was time to go flail-a-swingin'. I also learned the technique of getting the guys near the middle and quickly zooming across the border of the screen to reposition yourself. Those improvements got me to… wave 13 on my 3rd run and wave 12 on my 4th, which doesn't sound like much BUT LOOK WAVE 10 IS HARD OK
I have a couple of small suggestions for improvement. First, that 5% damage boost upgrade feels like it does very little. With its current cost, buying it will probably lock you out of buying the more specialized ones, so I never really felt compelled to grab it. I think it could be made a bit cheaper without breaking game balance.
Second, I'd like a more flashy visual indicator when you root an enemy. The text makes it clear enough that it happened, but you could make it juicier by adding a unique particle effect and/or making enemies flash repeatedly when rooted.
Third, a music track or two would be super nice! The lack of one isn't enough for me to dock a star on audio since the SFX are so good, but a full version of this game should def have some sick tunes to jam to.
I could definitely see this being scaled up a bit and put on Steam for a couple bucks (a la SNKRX). Amazing work, especially given that you only used half the jam time.