ok here's the battle plan! it was easy once i figured out his tactics though lol.
macalania temple:
use rikku or anyone with steal to steal all the potions the two guards have and pound them away they should have 1000 hp very low
then use steal of seymore and start pounding away at him. do
not summon shiva or any aos yet just keep you health high and cast nul spells to dull/repel the damages he will cycle through all the elemental spells but if he uses flare your outa luck. after a while when his hp gets low he will summon anima. now summon shiva and every time her ph gets low use blizard or her ice atacks on herself to get her hp back. you might want to use it after he does his overdrive though
Bevell:
cast nul spells and do no summon. he will just bansh them before they can do too much damage. kill the dam scorpion thing it does damage to him too so after you kill that off kill symore again
Big mountain:
he will be in some sourt of chiken leged contraption so make sure you have death and doom defence. cast lots of nul spells he will sometimes cast dispell and use full cure on one of your doom characters and keep wouding at him with people with lots of high hp
Inside Sin/jeht:
the will be huge lol cast null spells don't summon will jsut get banished
kill the scorpion or whatever keeping hi alive then pound away at him
null spells again and holy if yuna or lulu have it
hint: try to equip lulu and yunas weapons with the 1 mp cost per spell or no mp cost per spell equipment will save you since i had it killed off hevelesca with it too makes things much easier
kill him and he will be sent to far plane and never come back so say sayonara and kiss my a** seymore
lol hope that helps a bit!