Can someone explain what's going on? I've seen the whole thing and have no idea what's going on here. Why did their costumes get torn off little by little throughout the fight? Wasn't the blonde chick immune to bullets? How did she lose?

Why did main girl switch places with the blonde in the end? How did everyone end up surviving at the very end? It seemd like everyone had died but somehow it was revealed they survived. But how?

Why is the main girl and blonde fighting in the first place?