Was stuck on him a while, but then I happened upon a method to make it more manageable. I used a Strength/Radiance build and he was just too brutal with constant attacks and warping around.
The cheese is quite simple: in the arena there are some broken structures on the left side near the entrance. The ruins themselves are impassable, but you can use a tiny area where those ruins/rubble end to put yourself up against the wall.
Eidolon will still jump around and you will still have to evade, but some of the attacks will get stuck on the overhanging parts of the wall on the left (that you are hugging). Also his attacks will be coming from a more limited "cone of attack", giving you mroe options to evade. He will also come close and then you can whale on him. I used the AOE spell which damages a circle area and the healing AOE spell in the corner which also helped.
For me this made the fight more manageable and enjoyable - hope this helps some of you stuck on him. It's a soft cheese as you can still easily die to him, but limiting his attack cone helped me a lot.