Tied to the same fight as this trophy:
I have a few helpful tips to help you get this.
1) Don't use any magic (other than Invincibility if you have the Magic Tiara) to damage her. Instead, focusing on just the hair whip will help to plan this some.
2) Hopefully, by this point, you have both the speed and damage upgrades for your hair, so you should be doing streams of 8 damage with every hit.
3) The Mouse Dance (rewarded after completing Cape Crustacean (in other words, World 3)) and the Mouse Bite (found in Tassel Town post-boss) are required for this. While the dance is forced, the Bite is optional. With it, you will be able to do damage to enemies when out of mazes, but you'll be stuck with 1 damage.
4) Because of 2 and 3, go through one and a half to two complete cycles of hitting Risky as Shantae to whittle her health down some. Once you are ready, go into Mouse and remain there (until she departs via cannon). The choice places to wait for her are on either side of the arena, where she runs to after her jumps. Spam the Mouse Bite while being near the wall so you can: avoid her shots, avoid her feet, and avoid her sword swipes, all while still damaging her.
5) Despite my suggestion in 4, it will still take you a bit. It is better to be safe than sorry for this fight simply because of the boss trio this entire set is (and unless you want to keep quit/restarting the fight, going under her threshold is better than going over).