I'm posting this in response to my level 40 thread. I read there that with a level 40-45 party the game was beatable but the optional weapons should not be attempted...

Well let's put it like this I don't listen too well. I aquired all the optional weapons with a highest level character of 46. Much of this I blame on owning the GBA version(still waiting for Cronicles to arrive) but I also put this on good strategy as well(or at least paying attention well). So now for all you still trying to unlock these weapons I would like to help...

I'll start by posting one boss and I'll post another later...
(this strategy will favor low level parties but may be useful to higher ones as well)

First
Dark Bahumut:
60,000 Hp

Definitely the hardest of all the optional bosses. He starts off with Mega Flare, and with a party with a low level like mine killing most of your party members (Rosa and Cecil where my only two characters to live through the openning attack and they had very low health). Fortunately I noticed that he does not use the attack again unless you summon Bahumut, so my first piece of advice is DO NOT summon Bahumut unless your entire party has reflect on them (this would reflect Mega Flare back doing damage to Dark Bahumut) but I had at least one character with reflect on at ALL times (you can't be too safe you don't want to do 57000 damage just to get crushed by Mega Flare. After the first turn I had Rosa cast Full Life (Life 2 depending on the version) on Kain (for you it would be the dead character with the highest amount of Hp. I did this because Dark Bahumut then goes on to use reflect on himself and Reflect Flare onto your party(against me Flare did anywhere from 400-2000 damage). Have Kain use jump the entire battle (unless absolutely necessary) so you know some kind of damage will hit Dark Bahumut the entire time (2,000+ for me) and you do not have to worry about Kain dying. Try to have your Hp as high as possible so Dark Bahumut's Flare may not kill him/her. Have your strongest party member attack constantly while the other three heal and revive all dead when Hp is high enough your second strongest party member can attack as well. When Dark Bahumut's reflect wears off he will cast restore on himself (Heals 6000 Hp) and will then recast reflect and continue his normal destructive Flare attacks. Continue this strategy until Dark Bahumut falls it may take some time but is well worth it Cecil's ultimate weapon is great. Just remember this strategy is all about lasting out a long battle so many Phoenix Downs and High Potions are necassary.

Hope that strategy works for you as well as it did for me I'll be back with a strategy for another optional boss later.