its all about strategy and timing...
if you stay around the middle of the arena and keep dodge rolling while he teleslashes you, you can time it (3rd or 4th teleslash) so you can dodge roll into him before he casts sins harvest and reduces hp and mp to 1...
vortex helps close the gap...
but if you kno you cant make it in time....
get a potion or something ready and run to the other end, time it so you can use the potion as soon as he hits you with sins harvest...
also if you time your dodge rolls properly later when he does omnislash, you can dodge roll thru it unscathed if you start the dodge roll thru him when hes about to do his third slash before charging...
jump the same moment hes about the land the final blow on the ground and you will exit completely unscathed...
aeroga helps later on, i wouldnt recomment it earlier cos it just wastes mp...
and i beat him while level 60 with kingdom key

right now im level 100 with 36000 munny...