There is one skill which I work on with every player I coach. It doesn’t matter what standard or whether the player is an attacker or defender. I give every player the same advice.
Focus on ball placement.
This is the simplest and quickest way for every player to instantly improve.
You don’t need to learn any new shots. You just need to improve where you put the ball.
With better ball placement, you put your opponent under much more pressure. You get weak balls back. You force errors. You hit outright winners. Table tennis becomes much easier to play.