The level you are at does’nt really matter, but if anything its probably better to be at a low level with strong magic junctioned so that you have high health but are on a reasonably low level so Bahamut will have low health.
As you won’t have Eden yet (Obviously) the most effective summon against his high defense is a high leveled (60 or so) Cactuar because no matter how high the defense of an enemy is it won’t effect the damage done.
You should keep at least one character using healing magic at all times.
Start the fight by casting aura on Squall and hoping he uses lionheart, even though you said you don’t have Irvine on a high level, you could have ultima junctioned to his Hp so he has a high HP and have him aura’d, and keep him pumping pulse ammo.
Finally an effective method is to junction Kamikaze as an attack (Cactuar learns it) and when you get him to about 40,000 health then use it on him, as although it KO’s the character using it, it is still a great way to finish of weakenend bosses as it deals 50,000+ damage to an enemy/
If you need any more help or don’t have some of the items I stated then just PM me and i’m happy to help!