Okay, I have made some changes and fixes.
Compared to the original, you should find this VASTLY more playable. The attacks trigger better, and the enemies die better. The level is easily beatable but still challenging.
Now, you will want to spend some time on this to further refine the combat system and improve the game. You can tweak the hitboxes or whatever you want from here on. I just wanted to fix a few things and get you started. Normally, I would spend a long time on this step, but I did what I could in 1 evening.
1. Animation triggers were tweaked to not interfere with attacking. This means you can now attack while walking, jumping, or falling. I ended up just putting everything as a sub group under a condition that attack was not playing.
2. I changed right and left buttons to be held rather than pressed. Before you could walk back wards if you held both buttons or pressed one while holding the other. I think this works better.
3. I delayed the mirroring if you change direction while attacking so it attacks before turning. This is the way it was in the popular game "Rogue Legacy" which I have played quite a bit. Feel free to modify this if you do not like it. I thought it worked well for the game.
4. I got tired of not knowing if something was getting hurt or not since there were no life bars. For testing and debugging, I added a number health bar to the player and each enemy will spawn one. This is just for testing and can be hidden or disabled in the final game.
5. I moved the enemies HP to their body rather than their box. Mixing "for each" with enemies and hitboxes became a chore, so I changed it a bit.
6. I added some groups and comments to organize things a bit. I did not fix everything though, so continue to organize things. This makes testing and debugging MUCH easier. Trust me.
7. I removed all instances of the enemies' boxes, and put them in a container so each enemy will spawn a box of their own and destroy it as well.
8. As a couple posters suggested, I added some subtle tweaks. I sped up the attack slightly (to 15), and I added a screen shake when you hit an enemy (triggers when the enemy gets hurt). This did not take very long really.
9. I did various other little things like disabled some code that was bothering me. I have not fixed everything though.
10. For some reason, the enemy on the far right at the bottom does not spawn for me. Everyone else does though. Maybe replace him or figure out what his deal is.
In all honesty, I can't remember what all I did, but let me know what you think and if you have any questions.