How many times did you die on your first play through? I died in a fight with Seymour (third I think), against Dark Yojimbo (damn his Zanmato crap), The Dark Magus Sisters (damn their Delta Attack). For some reason, Braska's Final Aeon (in my defence I had quite low stats) and Penance had me about four times. 

and with just Tidus and Just rikku with no overdrives....for Sinspawn gui(one at mushroom rock) Auron use Power Break and then use Wakka and Lulu to take out the head with Magic and ball. and if you have Fira by Gui your seriously overleveled. Once head is dead switch Wakka for Kimahri and have Auron and Kimahri attack the arms and lulu attack the body with magic. heal after his puch attacks with potion/hi-potion and Lulu will need an ether in this battle
)