How do I beat this boss ?

I download an emulator to play ff6 and this slug creature keeps killing my party I never played ff before what do I do it hides it's head and uses this superpowerful attacks and nukes the field. I can't use my electric spell I can't cast banishment either.

I download an emulator to play ff6 and this slug creature keeps killing my party I never played ff before what do I do it hides it's head and uses this superpowerful attacks and nukes the field. I can't use my electric spell I can't cast banishment either.