Myrsh is a halfbreed necromancer living in a xenophobic elven kingdom. He kidnapped the elven princess and trapped her inside a gemstone. He is very powerful and has supersonic speed. He wields a thin sword that reanimates his enemies after they die. His sword can easily pierce steel. The zombies are controlled by a psychic link. However, he can only effectively control 50 corpses.
The elves don't have supersonic speed but they have elemental mages that control fire and wind.
My anti-hero cannot flee from the elves using his speed because the kingdom is on an island.
How many soldiers and mages you need to stop an enemy like Myrsh and save the heir of the elven kingdom, and how would they do it?
Notes: The only way to kill the zombies is by incapacitation or cut their heads. Myrsh wears chain mail under his coat. The elves are decent warriors and mages but they are no match for the young necromancer at 1 vs 1 combat. The elven warriors wear plate armor, a broadsword, a heavy shield as well as a crossbow. The mages just wear clothes.
The conflict takes place on a coast. Magic is limited by line of sight and physics
The elves need to be careful in the battle if they don't want to break the gemstone that contains the princess. If that happens, then she dies