Background: I'm an extremely powerful guy. However I've sworn an oath that if the protagonist's companions (of which there are five) can best me one on one I will give one of my five powers (fire, electricity, telekinesis, regeneration and time-stopping) to each. When the first defeats me I pass one power onto them; I lose the power, but receiving it kills them. The protagonist then has to watch (and the player control) the other four companions as they sacrifice themselves to take my power, making me killable and leading to the final fight:
There would be no theme music, because the finale is a one on one knife fight in space with me giggling and whispering the companions' names quietly over the radio.
I have a spacesuit with a jetpack and near limitless invisibilty (I just can't attack when cloaked).
The protagonist has a length of metal pipe she can use to block my attacks. Her suit is normal and will rupture fatally if cut.
We're both falling towards Earth, but only the protagonist's suit is designed to cope with re-entry.
Survive.
Because conventional boss fights are so last decade.