I beat it by doing nothing because my build included robot toy and too much armor for any of his attacks to break through.
The attacks of his allies are easy to deal with if you have at least 4 energy per turn (you need more in endless mode). Given the nature of the elder mage, armor and dodge are actually completely useless during boss fight. Robot toy seems like a great way to kill allies, so I guess that about covers the entire fight. Does robot toy trigger dodge?