Dyne
Making this battle difficult is that you have to fight Dyne as Barret with no backup. Due to this you will have to micromanage your turns so you can heal when you need to. Due to Dyne's high damage Barret's limit meter fills fairly quickly so use his breaks when you can.
Dyne is invulnerable to graivty so do not waste a turn trying to hit him with it. He cal also cause upto 300 damage in a single turn so keep your HP above 300 to insure you survive if Dyne decides to throw an S Mine attack after his normal one.
After dealing with this guy you get a Silver Armlet and a pendant that belonged Marlene's Mother.