Rennala is an optional boss within the Academy of Raya Lucaria.
This boss has 2 phases. For phase 1 you need to attack and kill the 3 smaller enemies that have the golden ball around them. Once you have done this Rennala will fall onto the floor where you can attack her. Don't get too greedy, as she will glow gold and then do a burst attack which will hit you!
Once you have defeated her, phase 2 will begin where you are in a beautiful environment with a moon in the background. She does various ranged magic attacks that will hit quite hard, she also will spawn a dragon that breathes loads of fire that spans the arena. To avoid all of this you can run quite far back, but ultimately you need to be closer to hit her. If you are struggling, you can summon in the wolf ashes which tend to stunlock her so she can’t cast her magic attacks.