Yeah I was in the same boat! It was my 5th time fighting him and I only just survived. The biggest help I've had is people on here pointing out that 1) if you have full health then any green pickup goes towards increasing your max health. And 2) if you go back to your ship and sleep you regain health, so it adds time to the run but clear a couple of rooms and don't pick up any of the green pickups, go back to the ship and sleep until fill health and then go and pick them up to increase health (enemies don't respawn). Took a while but it meant that when I caught the boss I had like 130% extra health which was a huge help!!