well, train on flan princess after beating zeromus, and until you are decent level. im level 99 all now ^^ (dangit, rydia was so hard to level up)
dont use sirens and use the bug, and youll get at least 1 addy armor (and eqeuipt it on your best attacker (unless its a warrior)
if a monster uses reflect, just use that big axe thingy from moon village.
theres a cheap way to kill him...use abels lance and tornado him by chance. but thats by chance, and its super cheap

also, if you are totally lost, use all of your grimores. they should do lots of damage.
also, rosa heal = evrey turn
and if rosa die =heal 1st priority
after beating him once, it gets easier and easier.
btw, try to have dragoon plate on kain instead of addy armor. its not as good, but resist just like addy armor, and you can give addy armor to like rydia and rosa.
hes not that hard guys....really...training on pink pufs really makes differance.