I'm so stupid. I can survive there too, I just forgot to buy second party member.
So, I'll buy Slow Learner (Theif) 1000/200, buy the 25 attack weapon (how many slots does it have), and I'll also buy the 30% lightning materia.
This may get complicated LoM.
Unequip the Fire materia of me (Bahamut)
Equip the 30% Lightning materia on me (Bahamut)
Equip the 25 attack weapon on me (Bahamut)
Then, equip my old weapon on SL.
Lastly, equip the fire materia on SL.
Then fight all turns in Sewers! (NOW lets see the difference!)