Thank you midnight ninja! I am glad you enjoyed the game and left a comment. Having more singleplayer (and other) content in the future would definitely be really cool, but right now my focus is on my other game Monster Battlegrounds.
I made the attacks quite a while ago, and there are some things I would even do differently, but I am still proud of how smooth the animations feel in the gameplay. In my opinion, the best way to create attacks it to start by creating the key poses. For example, when you are animating a punch, start by drawing the pose where the character is fully extending his arm, when you would hit the opponent. Once that is finished, then draw the frames before and after that. It is usually better to make the startup of the move take longer than the recovery after the main action, so for example: 1 frames getting ready to punch > 1 frame PUNCH > 3 or 4 frames of pulling arm back. It is ok to stretch the bodyparts a bit, even if it looks weird on a still frame, it might make the action look better in motion. Make sure there is a bit of variety in the timing of your different attacks, and take a look at real-life pictures or footage of martial arts, not just from games. There is also an online resource that collected all the animations of the classic arcade fighting game street fighter 3, I think you should not just copy those animations, but it will help you understand how certain body parts move and where professional animators squash and stretch their animations:
https://www.justnopoint.com/zweifuss/
It would be very fun if you released your own game! If you have an early build, it is ok to post it here in the comments to let me know, and if you want to I can give some feedback on the game. Thank you for commenting and thank you for playing!