For CC1V1 I feel like it rests largely on mind games - trying to guess which Pokemon your opponent will use, and picking the one you feel like will counter it best, or going through your pokemon, noting useful moves, levels bla bla bla. Even if you don't know what moves your opponent's Pokemon will have, you can make horrible guesses, and you try to pick something that covers the likely 2 or 3 pokemon they're going to pick - I mean if they have a magikarp it's probably not going to be chosen right?
It is mainly luck, and people at a higher rating are probably experiencing a degree of luck - but in terms of strategy, I feel like it's just really overthinking and trying to outmaneuver the opponent