Lucina vs Young Link

Lucina's normal attack range is longer than Young Link's. If you can keep mindful of the distance (and hopefully out speed him), you can win.

His combo starters are throws and down tilt. When he grabs, it has distance, so wait for it to fail, then come in and punish him. Down tilt has a bit of lag, so you can keep your distance in the air.

Don't camp him because he will win you. You should adapt and be more agressive. Side B is good pointing down is really good