I think we have to learn from lessons, books and other sources and then try our learnings in drills. The level must be hard to prepare us against the best players. I'm guessing 2000+ rated players will hardly make any mistakes in drills.
Drills are concrete examples on how to do certain things. Your opponent should not make mistakes when replying in a drill, so ideally it will always be perfect play, or potentially play that is the hardest to reply to (gives the best learning advantage).
I bought Dvorestky's endgame manual, and try to understand each concept. Key squares, opposition.... on so on. Then the drills I'm doing are doable even at level maximum. You need to study, this is the only way!