How strong is bahamut when he is in his cave in the underworld?

In the underworld, Bahamut has 45,001 hit points and his attack strength is 174. But his defence is 1 so I recommend that you use Reflect so when he blasts you with Mega Flare it goes back at him for roughly 3,000 damage. And use Rydia's Leviathan summon. Rosa should use Heal and Reflect. While Edge and Cecil should non-stop attack him with there swords. ...Later on when your on the moon trying to get Cecils ultimate sword, Ragnarok, you will fight Dark Bahamut who has 60,000 hit points and he starts things off with Mega Flare on the whole part. Usually it leaves 2 of your party alive, Rosa & Cecil. Heal up and use Reflect. He does head on attack you though and he casts reflect on himself so make sure to bounce your spells off your teammates with reflect casted on. (Your summon spells break through his reflect)

He as about 50,000 hit points and about 170 attack.

Bahamut has about 170 strength and 40,000 hit points. I reccomend characters between levels 50-60 at the minimum just to be safe against this guy. Also, casting reflect on yourself to avoid Mega Flare is a good stragedy.