if u can beat the first mission or higher he will be in the dark depths in hallow bastion

To find seprhioth unlock the land that never was then go to radiant garden go to the place you fought 1000 heartless then go to the place called dark deaths go into the big clear space then you'll find a guy with gray long hair press traingle on him and fight him WARNINNG! seprhioth is powerful be carful.I don't know any fighting tips SORRY.but good luck!

to fight him you have to equip allot of elixirs and level up your forms zo you can get an advantege i would recomend to level up all of your forms including final form

hsfh

all you have to do is do all the things in the computer and beat the boss and that and the will now be called radiant garden and than go though cristal fissure into dark dark depths and .......... will stand on the corner (hardet boss on game)