I haven't touched on that ground but all I can say is goto a place where fiends are hard and level up. Try to get Paine to master Warrior and get her to do Excalabur (which is a holy attack) and it can deal quite a bit of damage. Yuna should master her gunner ability and use On The Level skill which deals damage to an enemy according to their EXP. Rikku should learn White magic and cast protect on party. Alternatively, use the Helios Guard Garment Grid and it will let you cast protect. I use this tactic for fighting hard assed fiends.