Phase 1 is quite simple.

First of all, get rid of all skeletons, including those on the stair-like platforms. While doing this, monoliths will fall down from the sky. They will fall there, where the white line is pointing. Only stand still to land 1-2 hits on the skeletons and you should be able to avoid the monoliths.

The second thing you have to do is to break the large crystal in the middle of the arena. Move the heroine so that the monoliths will fall onto this crystal. Like I mentioned before, keep an eye on the white lines.

In the beginning, the witch will cast laser beams out of her hands. These lasers will go 3 times in circles around the whole arena. You can simply jump over them. She makes a "Ha!"-sound before she casts her lasers. So listening should make things a bit easier here.
Your first target will be the 3 larger crystals that spawned around the arena. Destroy them or they will become additional enemies.

Then attack the witch. She will change to a black magic, a bit similar to the magic of the skeleton mages, but way faster. She casts it in front of her, so move around her and try to hit her 1-2 times from the side. Repeat this until she drops some collectible crystals and the heroine says "I just...want to rescue the witch!".

When this happens, quickly run to the outer wall of the arena and keep running alongside this wall. The witch will cast a lot of crystals at you. But she won´t hit you, when you´re running this far away.

This phase is quite easy. The witch will use an obvious attack pattern with her swords. Simply move around her and attack her from the side.

A few hits later, she will start to charge at you. Go a few steps away from her and wait for her to charge. Dodge it and attack her from the side.

At the end of this phase, she will drop collectible crystals and fire crystals at you again. So do the same thing as before and keep running far away from her.

In the last phase, she´s trying to crush you with a crystal hand (two hands in the third spell).

Stand still and wait until the hand is falling down. The witch will make a "Ha!"-sound a second before the hand is coming down. Then quickly move towards the witch (the impact will be behind you).

When the hand is on the ground, climb it and hit the witch.

Do this 2 (or 3, when you´re not fast enough) times and you she will be defeated.