Not always. That will only work on machina. Those are mechs. Use Lulu. They are weak against water, so use her Watera or Waterga if you have it already. Auron is also a good choice bacause of his high Hp and strength. The last characer should be either Tidus or Kimahri depending on who you leveled up more. Attack with all your might, and switch in Rikku to use Al Bhed Potions. (That is, if your Kimahri doesn't have Use like mine did.)