I will post a video if you need, I will also post here a brief comment on what to do. That boss is a bastard, you are not alone ;)
I can start telling you that you may check runs from individual levels and also from the glitchless run category, so that you can see how me and others defeat the boss.
As you can see, the boss can kill a run easily. This game is far from optimized from a speedrun perspective and you should not try to get a level frame perfect. Start by just trying to get clean runs, then you should try to dive deep into perfect corner cuts, jumps etc...
Another thing important is to understand the timing. When you hit the boss, there is a cooldown in which he blinks and is basically immune to damage. If you don't wait enough time and try to damage him, you will just die. You don't have to wait the blinking effect to stop, check my run to see this in action.
With good timing, you should give him 4 hits before he starts going to the left side of the map. You can defeat him before he reaches the left side, but it is not that easy.
Due to the recent discovery of glitches that allow levels to be skipped, found by @Nuno137, which you can find here:
The rules for IL have been updated. Using this glitch in individual level runs is now prohibited. The glitch can still be used in An
