sos for the delay in nswers but to defeat that guy, attack the guado guardians and kill them, then summon ixion and use aerospark. if he kills him, so what? summon another aeon, attack (an elemental aeon is best) and attack. if you survive, heal yourself and attack again.
then have tidus cast haste and him and your strongest attacker and yuna in your party. have tidus attack, your strongest attacker attack and yuna cure. eventually, he will be dead and you will be victorious and if not, u need to train up more