You can only attack bases within five levels of your fleet level, so it is unlikely that you can create a fleet powerful enough to attack a high-level player's base. That player can build low-level fleets to attack your base though.

How do you deal with this?

You build a fleet designed to crush their attacking fleet and destroy them until they get tired (or embarrassed) of losing to a low-level player.

Then you level up and get revenge.