Ethanuel1
First, go to the castle and take it over. A while after you do that (you can repeatedly use the magic clock to skip a bunch oftime), Merlin will challenge you to a battle. After you defeat him, your Magic score will increase by 1. Second, go to the Witch's Forest on the right side of the world map. In there you will find Polyquin. After interacting with her a little bit, there's an event in her room that lets you drain some of her magic and increase your Magic score by 1 again. You can actually do those in whatever order you prefer, but taking over the castle as soon as possible is usually a good idea because Merlin, the Prince, the Princess, all won't show up until after you become King, and there's other events as well that involve you being the King.
If you don't have the maximum Magic score, you will need to do everything almost perfectly in order to beat Meradis. With higher Magic, you get 5 Barriers, and more chances to mess up. And even if you do poorly, you'll probably make it to the final phase where you can heal before finishing her.
Some additional tips:
When she is creating after images, the goal is to hold the arrow key in the direction of the Meradis version that has bright eyes. The two dull eyes are fakes. Up or Down works for the center, either is fine. You must HOLD the key, not just press it.
Aside from that, surviving her other attacks involves making choices about how to respond. Some of them are callbacks to mechanics from other battles. Like for example, if you remember the mechanics for the battle with the Harpy, you'll know when to use Ice or when to use Lightning magic.
There's a section in the
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on battles, you can get all of the correct choices for all of her attacks there.