Just one question

How can I catch a pokemon if, let's say, I have only high level pokemon and I want, for exaple, to catch a Pidgey? Because I can't lower it's health, just one-hit-kill it... Please be kind and help me... Thanks in advance.

You can use a high level ball or you can catch one of the pokemon at the very beginning next to the starter town without hurting them at all. Then train it up to the level of the pokemon you intend to catch.