The unexpected outcome is just something else to think the boss without HP would just be yeeted to the oblivion of the portal wherever it will be opened up. That alone is big and for Guts to suddenly do that, was something else too.
"I didn't expect you to kick the giant back to his own realm."
"Eh, it has to be done. How can we pass through if he just remained in a corner and keep on crying without leaving? And seeing that he is not even injured from our attacks just made me quite sure that he is invulnerable. You just hit his weakness and he was "defeated" which I would say to be quite lucky of you. Now close it before the guy come crashing here once again.
"Alright."