well in my opinion the three candidates for super boss are not that good.

two of the bossess(SPOILER) Belzebub and Tiamat are quite easy really.

while the third (SPOILER)Astaroth is very difficult but still just slightly weaker then the last boss.

i,d say to be sure of beating these 3 bossess, have 4000+ hp with like 13+ masterey on best weapons and 10+ on magic like ultima. of course u can beat them weaker then that but thats very useful. i also found bacchuss wine useful against (SPOILER)Astaroth if you dont have very good spells.

just use the training trick and your be fine. ffII hardness comes from its annoying huge dungeons, not its bossess.