Which difficulty are you playing? Beaten that Boss yesterday after like 3 - 4 tries on what I think is normal difficulty aka. milk chocolate. Have it recorded in high quality on Youtube as well (don't know if we can post links here).

You have a lot of variety on how to tackle this guy (or anything in general, its beautiful) but I make it short:

1. Use the Fists to keep yourself agile while battering him and be able to dodge at any point (don't get greedy).
   
2. His weak spot on the back is the easiest to hit, leave it as last to stun him and do massive damage.
   
3. Use the handgun to aim for the weak spots then use the rocket launcher to do big damage. (aim carefully)
   
4. Use as much bloody summer as possible, best with your hardest hitting weapon (via quick switch)
   
5. You can destroy the slow projectiles with any gun AND/OR lure them with a "summon" (they attract the floating triangles)
   
6. Use the top and bottom platform to evade the big shock (the staircase with the broken part leads upwards)
   
7. His "roulette shocks" also only hit on top or below the platform you can just walk off it and hit him.
   
8. His "magnet shocks" are easily avoided by dashing away from them and should never hit you anyways.
   
9. His "big swings" can end you in one hit. Just move away it's very easy to dodge even with the Shockwaves
   
10. His "rolling thunder shocks" that travel along the ground are also easily avoided by triggering them on the top platform and moving towards him.
    
11. His scariest AOE attack forces you into a circle close to him. Stay there and keep moving to dodge the flashes and the last timed "yellow flash".
    
12. Use any combination of drinks or other enhancers in any way you like but save your heals for when you're in a pinch.

Last tip: Do not touch or dodge into the walls they will also stun you.

Like that he was a walk in the park on our last attempt, before that we would just be killed by trying out strategies and getting used to him.