Bahamut (Arabic: بهموت Bahamūt) is a vast fish that supports the earth in Arabian mythology. In some sources, Bahamut is described as having a head resembling a hippopotamus or elephant.
Source: http:/en.wikipedia.org/wiki/Bahamut
where are you at right now? if you're done with bahumut, the pharos are the best place to lvl.
Each summon has their Upsides and Downsides. All Rounder: Bahumut If you have alot of money...: Yojimbo (You must pay him money for him to do certain moves. Do not let him die, or there is a chance he will run away and never be summoned again) Letting the Aeon's decide: The Magus Sisters
First of all have all your peoples over drives at ful. (this will help you cut Seymours HP down a little). Next you need your aeons at full HP and your people at full HP (make sure your aeons have their overdrives filled up also. 1) Summon Bahumut-Valefor from Bahumut to Shiva to Ixion to Ifrit to Valefor (make sure one fo these aeons have no overdrive so you can use yunas overdrive0 than use their over drives but after each over drive you use seymour will banish them from the battle killing them so you got one shot with their overdrives. The aeons overdrives should be able to cut his HP by half or more and the mortabsorber will absorb 4000 HP which will help you. 2) use trigger commands with Tidus, Yuna, and Auron. 3) use bio on seymour but if you havent used the shadow gem use it it h=will cut his current HP by half for a quicker battle. 4) use overdrives. first use aurons shooting star overdive it will deal most damage. then use Tidus' slice and dice. Then WAkkas element reels. Lulus overdrive. followed by rikkus and khimaris(tidus kimarhi and auron and wakka should have the battle mode warrior so if theyre overdrives filled up USE THEM) 4) use special abillities and you should win in a couple of turns at least 15-30 turns it will take 5) you should also have weapons that deal great damage. 6) before step 2 make yuna use reflect on tidus than have tidus use provoke this will also help making seymour use magic and hurt himslef 7) win its easir said than done though it took me about 5 tries to actually do all of this hope this helps you beat the game : D ^_^