I think it would be a bit like the battle system from "Clickertale" (an Undertale Clicker game, if you couldn't tell)
If you don't know how the battle system from that game works, attacks move around the screen and you must dodge them with your mouse. It's simple to understand but can still be used for some pretty interesting attack patterns. Now, in Clickertale, you have to click on the enemy to attack them. However in boss-fights I think it should be more turn based like Dn'D, with basic options like physical attacks, magic attacks, healing items, healing magic, along with standard Undertale/Deltarune options like actions, sparing, and pacifying tired opponents. (All the magic and actions are all part of one option which opens up a mini-menu) There can also be options to play with WASD and the arrow keys, this will be important later. Ok it's later. Some bosses have to ability to swap your controls, (when I say "swap your controls" I mean from, say, WASD to Mouse controls, not inverting your controls) This can be used very dangerously and is a powerful weapon for some bosses. Also like Undertale, there could be some "soul" types. Which could also put a spin on some boss-fights.
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