Just wanted to share this strategy I found.
First, it may seems a bit awkward pairing the mana stealing Onatel with mana blocking Proteus, but with the correct timing you’ll never have to worry about event bosses and their specials ever again!
The trick is as simple as it’s genious:
Make sure both Proteus and Ona is charged for boss round. Fire Proteus first, and make a tile move. Then fire Onatel. Mana is still blocked, but at these stages, Ona steals very little anyways. Then, when Proteus’ special wears off, Onatel’s skill is at stage 3, and the bosses gain only 25% mana first round, and none on the last turn. Make sure to match purple tiles under the effect to charge up Proteus again, and start over (Onatel charges herself, unless you’re very unlucky with your tiles).
I put Ariel in the mix to strengthen mana gain, but you can bring mana pots to always make sure Proteus is ready to go again!
This works great. I finished last level on legendary without a single special from the bosses. Alice was closest with about 75% mana gained in just under 5 minutes of gameplay.