Nice one! I remember Smash TV, can clearly see the influence! The ensuing chaos when the whole arena is filled is really entertaining, and the screen resize is very welcome, it all flows really well.

I would've love to carry all the weapons, I often felt like I was missing one. Overall it's a fun game with a solid gameplay. Great job!

Great job! This was a really inspired game with nice art, sound, and game feel. I could tell after 3 playthroughs your level design was pretty thoughtful, and intentful, and does a decent job of self teaching the game mechanics.  I liked it a lot.  The animation of the assigned weapon is nice and the art/ui is simple and cohesive. I have some feedback

For accessibility, it might be nice to make the arrows work for left-handed players too. I also would consider playtesting your game with a color blind simulator to see if it still looks good under that condition.  Add a pause screen, especially for those quick alt-tab moments, and a volume slider. The enemies kinda remind me of pig noses, in fact it was impossible to get out of my head. 

Maybe consider adding a custom cursor for additional polish. You could probably add a bit more impactfulness when the play is hit by the final blow, such as a flash or bounce and scale effect, or a . Adding some color to the main menu could highlight the elemental resistance thing nicely.
And on the "wrong element," maybe think about switching it to "same element?"

I noticed you can pickup items you already have and can switch to an empty weapon slot. 

This was an amazing game, great job.

There are a LOT of really interesting ideas here.

The slimes splitting if you hit them with the shotgun, unless they're frozen? Fascinating, caught my interest immediately. The different elements and needing to be careful how you used it was pretty neat, but it seemed like I could only carry two guns at a time, so that worked somewhat against the spur of the moment picking. On the whole, this is a really great entry, I think you should be proud!