(I'm a big fan of animes as well although I'd rather watch them in Japanese)
Its burning grip TELLS ME TO DEFEAT YOU! TAKE THIS!
I believe "Its burning grip"
could be translated to something like: Seu punho ardente ( flamejante/ em chamas/ chamejante).
The last animation I watched in English was The Legend of Korra which sadly wasn't as good as the Avatar: The Legend of Aang.