Equip Barret with a long range weapon and put him in the back row. Attack with your gun each turn your HP's are above 200, but once they drop to that point or lower have Barret heal with either materia or potions. It's not necissary to cast magic AT ALL during this battle, so only go in with what materia you need (such as Cure, Enemy Skill, etc.).

Keep the pressure up on Dyne and use Limit Breaks whenever the oppertunity presents itself.