Since I was explaining it to myself, the explanations seemed easy enough, but it didn't survive contact with players, I guess.
Shield is simply shield, reduces damage you receive by the amount between (). Contact moves still work as having hit, so it's not very effective against Dracula and Medea, whose abilities trigger when you are hit. It is sure to work, at a small random RPM cost, but can't stop contact skills
3DI is a parry move, the hardest to hit. You have a number from X-Y, and when you use it, you roll a D100. If your result is between X-Y, the parry succeeds, nullifying damage and absorbing the enemy's RPM. (Some enemies are pure DP and low RPM, so it isnt ideal against them, so the risk is not worth it.)
Flow is evasion. You have a % between your (), the enemy rolls a d100 + accuracy value, and if they surpass your evade value, it hits. The same applies to you when the enemy defends with those.
Resonance is preparing for the defense turn, increasing your dodge rate, the range of your parry and giving you a charge to pay for the shield or prepare for your next attack to be stronger. At first it must be hard to choose what to do, but as you get to know enemies and see what their action will be, plus get used to their gimmicks and know what is better not to do, there are about two options depending on your build that makes it easy to cheese every boss. When you get their gimmicks, they're all beatable without any upgrades, I balanced and tested them all to be so. But asking a player to get that engaged with the mechanics is a challenge in itself, so I can understand if it just looks confusing and or frustrating at first. (secret function that the tutorial doesnt tell you and I kinda wanted you to go OOOH when you found out: Resonance nullifies the dance skill of enemies that have them, so the strategy against medea for an example is to use resonance to not let her poison you, then going for an evade to not let her hit you and nerf your skills. she always evades and is a sure hit, but once youve unlocked the actions of other enemies, you can choose to use resonance whenever you get the message theyre evading)
Thank you for the feedback! I'm glad you liked this world, as I came out pretty proud of it when I finished it!