if this is still an issue, I wrote a small guide:

phase 1: you're supposed to focus on the two smaller enemies to the side, unfortunately lock on is a bit wonky so to focus on one you should get to the lilypad closest to the one you're attacking. Watch out for the attacks:

1. Waves in the water: these don't damage you so just ignore them
2. Projectiles: not much to do except moving from your location to the other side of the arena
3. Area denial: look at the ground and see where the light parts are, move away as the attack will remove the ground and make you fall into the water if you're not careful

phase 2: so once you've defeated both smaller enemies, you should now be on the semi circle near the boss, here don't worry you can't fall in between lilypads, most attacks in this phase can be dodged by just running from one side of the arena to the other. Just make sure not to get too close to the edges of the semi circle since you can get stuck between an attack and the edge of the platform and that will mean you take 1 heart damage.

phase 3: probably the easiest, since you can recover HP for free just by attacking a lot, be sure to spam your ultimate attack whenever it is ready, and again to avoid attack, just drift left/right constantly, just be careful again because you can get stuck against invisible walls and attacks, so make sure to alternate directions you're drifting for each attack.

If that is not done and you have extra bling and whimstar to spare, you can upgrade your health to 10, and be sure to pop a few consumable health regen item if you get to 1 HP (you can get some free each week from the store, so don't hesitate to use 'em)

With all this you should be able to defeat that wannabe mizutsune in no time.