itch.io

One big thing is not to use the "Attack" action when it dares you to attack--the counterattack is harsh.  (If the kobolds are with you, they'll warn you.)  Use spells, items, or defend instead.  

Otherwise, use damage-over-time effects and keep healing your team--I generally like to use Forest Song, Spring Water for mana regen, and have Lyric heal as needed.  The HP keeps fluctuating, but it does eventually come back to manageable levels.  

Make sure you've gone into the Class menu and selected some chimera genes for each of Hero's forms.  I don't know if there's an in-game instruction to do this, and it's a significant buff.  

Remember that you can always go back and grind for more levels if needed.  I was level 30+ when I faced Chaos; that might be overkill though.