not the last boss but is one of the many in the final part of the main game (for you GBA version players) but yeah he does counter attempts to avoid his earth attacks with 100 Gs
I remember this, you keep your characters floating and use Reflect Rigns, like StriderTuna suggested. I think you could just keep one guy floating and he'll keep 100 Gs'ing you until it works or you kill 'em.