seems ~15 musical attacks are enough and then you can kill him before his turn! i did rebuild my characters to max strengh, not recommended on a usual gameplay!

This one is tricky because the mage minion who spawn on the right of the first wave will heal him of 750hp at his turn so you can 1/ try to finish him before this minion turn or 2/ CC him with an ice bomb or something else
When he use his big delayed AOE attack simply push him with the Dwarf, Barbarian, Priestess or Wizardess spell

To unlock this secret fight, just follow this very well done guide: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2242450353