The earliest point in the game to get Bahamut is on disk 3 once you have acquired the Ragnarok. Before setting out to get Bahamut make sure you have some good junctions and abilities. Don’t worry about your characters levels as almost all monsters and bosses in Final Fantasy VIII (Bahamut included) level up as your party levels up, so in essence the stronger you become the stronger your enemies become. Meltdown, meteor, triple, flare, holy, ultima etc are good to have before setting out.
Once you’re ready set out to the deep sea research center which is located in the far South West of the map (bottom left hand corner), you can’t see it on your map 7.62×39 surplus ammo. Once you find a small floating island, land the Ragnarok and walk inside. You will notice a glowing, blue core in the middle of the room. Walk toward it but pause when the glow intensifies, not doing so will cause you to enter a battle with Anacondaur’s and Blood Souls. Encounter None and Encounter Half have no affect unfortunately.
Once you reach the core you will be given an option of what to answer, choose “It is not our will to fight”. You will now enter a battle with a ruby dragon, they can be quite tough so be careful. If Quistis is in your party kill it instantly with a “degenerator”. After you have beaten it you will be given another option of what to say, select “Never”, you will enter a battle with a second ruby dragon. Once defeated you will be given a third and final question to answer, select the third option (it’s blank, you can’t see it). After a bit of dialog you will fight the big boy himself, Bahamut.
Hopefully you took my earlier advice and have equipped some decent junctions and abilities. Begin by casting meltdown on Bahamut then aura on your best characters. Limit breaks like Irvine’s shot (AP or pulse ammo if you have it), Zell’s combos and Squall’s Renzokuken work well. Heal with X-potions or Curaga spells or Quistis’s white wind, her mighty guard works well too in buffing your characters up. His HP is dependent on your party’s average level so the length of the battle will likely be dependent on what level your characters are at. Eventually he will go down and you get this great GF as well as his card (which converts into 100 megalixers using card-mod).