Focus Punch is one of the most powerful fighting moves you can find in the entire Sinnoh region. This move has a whopping 150 power and is guaranteed to hit (as long as the user isn't hit while charging). It will take some time before you can get your hands on this powerful move. Here's where you can find this TM in Pokémon Brilliant Diamond and Brilliant Pearl.
Before you can get your hands on this move, you'll first need to learn Rock Smash, Strength, and Surf. This means that you will not be able to get this move until you have beaten all the gym leaders. Once you can use all of these moves outside of the battle, go to Oreburgh City and travel west to Oreburgh Gate cave.
After entering the cave, take the path north to the stairs. You will enter a large room below. Travel south until you find a body of water. Use the surf and travel west to the left side of the water. Here you will find a veteran trainer blocking the path to a ladder. Defeat the trainer and go down the stairs. There are two elements here. The north object behind the boulder contains the Focus Punch move.