How exactly did Luthor kick Power Girl's ass so quickly?
She's a kryptonian. She has all of Superman's powers, and should be every bit as strong, durable, and difficult to knock out. And yet she was defeated by a single blast while Superman took several and still managed to win. Why? I understand there might be a difference in skill, but not in durability.
- It's never made clear how Kryptonian super powers actually function and given how poorly Supes seems to do whenever he runs up against someone with martial arts training from Krypton it seems plausible that their strength is some kind of multiplier, not an add-on. Meaning Superman is 1000x as strong as an adult male with his body structure is and Power Girl is 1000x as strong as a woman with her body structure is on average. She doesn't have as much durability training as Superman, and she hasn't had to deal with the effects of kryptonite radiation quite as much, so of course Superman's better equipped to handle Lex. It's worth noting that according to official canon Wonder Woman is the strongest female character in the DCU: and while it varies from time to time Wonder Woman is rarely depicted as being on par with Supes himself, and Power Girl and Super Girl are beneath her.
- There's some other things: First, that blast caught her off guard. She wasn't ready for the blast to begin with. Second, we don't know if that was the only thing Luthor did to her. A decent chunk of time passed between Luthor blasting her and Superman confronting him that it could have been more. Additionally Superman was getting curb stomped as well until Batman took off in the rocket, only taking three direct blasts (with just one doing a lot of damage in of itself) and getting decked with a few charged punches before Luthor just didn't bother charging. After the rocket took off? Superman was pissed and severely hopped up on the kryptonian equivalent to adrenaline. When he gets that mad he's pulling off feats that make Darkseid wonder if he didn't go too far. Finally, Luthor wanted to break Superman while Power Girl was just in the way. Luthor would be willing to use lower power attacks on Superman to keep him alive long enough to break him down before killing him. Would have worked too until Superman got angry. At that point Luthor didn't have anything that could stop him.
