it is possible to beat him at 54 cuz u can dodge/block all of his attacks although having more hp wouldnt hurt
here was my strat agaisnt him. u can make your own strat with practice
for the fight de-equip all combo/air pluses and other attack enchanments (i think thats the word). the abilities u do want are slide dash, ( makes u rush to enemy when u are at a distance) combo negation (if u have, fewer the hits the better), guard break, aerial finish, retaliating slash, combo boost, aerial boost (if says it does more damage on the finishing hit depending on the number of combo hits. i'm not sure if it does more if u do alot or afew but it doesnt hurt to have it on) second chance, once more, and finishing plus.
when u start the fight, press triangle to block his attack. next, he'll be right next to you so unleash ur combo. (the 2nd to last attack of ur combo should be guard break which pierces through seph and knocks him up in the air. the last hit knocks him away.) he unleashes a 5-6 combo after he gets back up from the combo so run away as soon as u finish ur combo. after seph finishes his combo u slide dash him and do your combo. repeat the above.
from time to time he will teleport to your and hit u into the air. once this happens he will air combo you-tele-air combo-tele. retaliating slash comes in here after his first combo. it interrupts him and this also gives u a chance to unleash ur own air combo (2nd to last move is aerial finish and last move is finishing plus. if u're high enough above the ground u can unleash a 2nd air combo) after u finish ur air combo and both of u are falling to the ground, u can move Sora forward to unleash the 2nd one.
Seph when 8 or 7 life bars:
he will unleash dark orbs which u can reflect with reflectaga. he will also unleash his flame pillers (from the first game) it pulls u in so glide or run away. remember that move at the start of the battle u blocked with triangle? he does that form time to time.
Sin harvest is what will probably kill u. if u are locked on to seph and he disappears and u are not attacked 1 sec later, that means he is using sin harvest. he will glow red so rush over to him quick with a double jump or glide and a combo is enough to knock him out of it.
Seph when 4 or 3 life bars:
err he turns red. i dont know if this does anything. continue attacking him and YAY u WIN!!
btw:this is my strat i used for proud mode. from what i heard he is extreamly easy on other modes. hope this helps elixers and hi-pots needed of course.
i choose defense (when i was roxas) so u might have different abilities
others have said u can use trinity limit, i did not know this (could probably be helpful)
u can keep playing after the final fight but the only extra things u can really do are finish all tournments (8 cups), beat seph, gummi missions
