Not a very hard boss if you know what you're doing. In the first part of the fight he will take some hits then fly upwards and some connected balls of light will fall. These can be destroyed by hitting the centre orb but you also take no damage if the centre orb passes through you. ie you can stand in the middle of the beams.
After a few passes like this the boss will fly down and launch a triple beam attack that rotates around the screen. You can avoid this by walking around and using the ledges to the right to jump up on top of the boss (this won't hurt you) and just move around in a circle along with the beams as they move until the boss is defeated.
Using the above it's possible to take no damage on this boss easily enough (as you'll be looking to save as much health as possible for the final encounter).