The stage 6 boss ruins the game completely. It's completely invincible when it uses its default attack. It can only be harmed when it decides to use a different attack. Its invincible attack drives you towards a wall of spikes that instantly kill you. You are not able to position yourself so that you are not driven towards the spikes. Your heavy attack does not work against this boss for no reason. There is nothing that your eagle can do. The boss only registers damage as being done to it when it enters its "recovery phase". It will only enter its recovery phase if you hit it with the slowest part of your basic combo. It almost always just does its invincible attack before it lands. You can do an uppercut to knock it out of any attack it does and push it back slightly, but it seems to store frames on its invincible attack so that it can bring it out quickly the instant you stop uppercutting it. Actually beating the thing requires more exploit than skill.
At least the sequels are better.