Yeah, I very rarely manage to do enough damage to kill him that quickly, but it's the only way to get the Speed Run achievement. It takes me a lot of tries. You have to be either really good, or really lucky, to hit him enough times during the homing phase to make it possible. In my case, I'd say I'm getting better at avoiding him, but hitting him enough times requires a ridiculous amount of luck. Since he goes in a random direction at the start of the homing phase, I just go right at the start of the phase and try to react accordingly, but I'm by no means good at it. In any case, even if you get lucky and manage to damage him enough, then you need a bit more luck to get the last few hits in during the circle phase where he fires gems at you. I died twice with him having 1 or 2 HP left the other day. So annoying!