I’m actually not convinced YA high fantasy is a tough sell—if it’s accessible. Look at books like GRACELING and SERAPHINA.
A high fantasy query should be like every other query. The most important details are who are your main characters and what is the main conflict. You can include some worldbuilding details in the query, but don’t let them bog it down—you want the agent to know it’s fantasy but not feel confused by the world.