Really satisfying and fun! Nice job! I think the core of the game is really solid. This could honestly be fleshed out into a full game like Devil Daggers. I think it is really great.
A few small comments. I don't like that upper ring surrounding the arena. Seems like it only exists to block the high flying minions that should be dying, which is frustrating. Saving minions that should die and then you have a low health minion on the far side of the field you have to eventually go over and kick again.
I think the kick hitbox needs to be bigger, like, a lot bigger. It should be capable of sending all minions in a six foot area flying. It's a little annoying to go to kick someone that seems pretty close and have your kick miss. Plus I think it would be hilarious to send like four minions flying at once. And juice up that kick, baby! The voices on the minions are amazing and hilarious, but the kick itself is just a womp sound. Let's get some screen shake, sound effects that scale based on how damaged the minion is... I want it to feel like you just got a check for a hundred dollars in a card from your grandma every time you kick a minion across the field.
I didn't find I used the slow-mo much. I was able to dodge enemy projectiles just kind of strafing around (Not that I see that as an issue, the strafing was fun). I think that could be like a dodge roll feature instead? Or something? I think it could be cool to fly at an enemy and then POW kick them.
Lastly, the health was really hidden. I was never clear on how many health packs were actually in the arena. Why not make it so there's 4 single-use health packs on the edges of the arena that are all visible from the center of the area? Give the user a nice at-a-glance of their resources.
Okay I only have a lot of comments because this core is sweet and you should keep working it! Really nice job.