Strength is one of the hidden moves that you must find throughout Pokémon Brilliant Diamond and Shining Pearl. This move allows you to push boulders out of the way and clear pathways. Before you can use this move, you will need to defeat the gym leader in Canalave City. This is the sixth gym you will take on, but you can get the TM as soon as your second gym battle. Here is where find this TM.
Before you can get this TM, you will first need to make it to Hearthome City. By the time you reach this city, you will have fought two gym leaders. After dealing with a few tasks in Hearthome, you will be tasked with going to Veilstone City. To get there, you will need to take Route 209, which is east of Hearthome City.
Be sure to stop at the Pokécenter before heading out since you will be facing off against several trainers on your way. Follow the route until you find the Lost Tower. This landmark is near the end of Route 209 before you enter Solaceon Town. Go inside the tower and head to the top floor. When you reach the top, you will find two old women in the center of the room. Talk to the lady on the left to receive the TM. Talking to the lady on the right will get you the Cleansing Tag.