Self-explanatory. For example.

Queen Chrysalis:

Nightmare Moon:

Had this play in my mind when Twilight dealt with her in episode 2.

Sunset Shimmer (Demon Form):

Fierce Battle is likely for Discord. Perhaps Weapon Raid for Tirek. All I can think of for the Dazzlings is Leblanc's theme from X-2.

This theme I pick be suitable for Cozy Glow, Queen Chrysalis, Lord Tirek is: Emperor Mateus Final boss theme from Final Fantasy II

King Sombra:

7787279
I do like that suggestion. But I did think of something else...

Legion of Doom (Chrysalis, Tirek and Cozy Glow):

Cozy Glow:

As suggested by Omegamorph...

Tirek:

The Dazzlings:

Midnight Sparkle: