You're incorrect. Aside from using the Amazing mighty form for finisher purpose only when fighting Go-Gadoru-Ba in 46th eps, Godai used Amazing Mighty form again in the 47th eps. when he was defeated by the final Grongi, N-Daguva-Zeba. But we, as the audience, only seen the losing battle from Godai's view/field of vision, where he was overwhelmed by the final Grongi in that eps. But, it was clearly explained that Godai used the Amazing Mighty form at the time. Rewatch the opening of the 47th eps.