If you want to be the best trainer of Pokémon Brilliant Diamond and Brilliant Pearl, you'll need to get the right items to power up your team's attacks. The Fist Plate can drastically help your Fighting-type pocket monsters in battle.
The Fist Plate is located on Route 215, which is east of Veilstone City on the map.
Keep following Route 215 eastward until you see two wooden bridges. The martial artist should be in the upper right with an object ready to grab next to him. Go through the tall grass and on the right you will see a tree ready to be felled. Using your Poketch, use the hidden Cut move to clear the way. Go right and you will find the Fist Plate after fighting the martial arts expert standing in your way.
The Fist Plate is exceptionally useful for anyone using a Fighting-type Pokémon on their team, but the item must be kept. To do this, press Y and go to "Bag". Now scroll right to the "other items pocket". You should see the first dish listed. Press A on the item and select "Give to a Pokémon". Now, your battle-ready monster is even more powerful.