My PCs have reached X power level where they can just blow through everything I put in front of them. What do I do to challenge them?
Now this is a natural stumbling block for folks to have as a GM but it isn't really a problem for two reasons.
First, as the GM you get to make the challenges and aren't beholden to anyrules for it. If nothing else you can always clone the PCs, add some XP, and put that in front of them as their new obstacle. I don't recommend using this technique often, but watching a player claim how "broken and unfair' something is then explaining that all you did was clone their character sheet can definitely be a cathartic experience.
Second, and more important, is that if you are interested in telling a good story with your game (and if you're not, that's cool) you're going to get a lot more mileage out of focusing your concerns and presentation around what the PCs will do and not what they could do.
Could your PC just shoot the bad guy in the head? Sure, why not? Not very interesting right. But will the PC shoot the badguy in the head if the badguy is offering him money? Will the PC shoot him after getting the money? Will the PC try to help the burning village?
For that matter, what won't the PCs do? How do you confront them with that?
This is where the tension and drama in a story come from, and the whole thing is explained much better in the video that first explained this concept to me, and which I've linked here before but am going to share again in the hopes of exposing more GMs to this thought process.