Persona 3 Portable

This achievement requires you to win a battle with one of your party members equipped with an item of clothing or footwear that is acquired from a rare treasure chest and where the item in question is shown being worn on screen. You only have to have one such piece of equipment in use for this achievement to unlock, and you can acquire the gear on your very first venture into Tartarus, where the first block of floors up to 16F is open for your exploration. Two pieces of gear from rare chests can be found:

Shirt of Chivalry (can be worn only by Junpei, raises DEF to 46 and strength by +3)
Rugged Geta (Can be worn by all male characters, raises EVA to 14 and endurance by +3).

Now, I don't know if the Rugged Geta would count as qualifying to this achievement (because I didn't get these until after unlocking the achievement with the Shirt of Chivalry.) but the shirt definitely will work for you. Once you get the shirt, equip it on Junpei, find a Shadow to engage in battle, and win, where the achievement pops on-screen afterwards. Happy dungeon crawling!