The sentence said by a boy who looks like being angry because of something.
なんでもねえやこっちのこったい! なんにしてもおれはでねえぞっ 一歩だってこの ひみつ練習場を...
I have found that several Japanese forums told that「なんてこったい」 means "What the hell" or "Oh my god" but I'm not sure what just the word alone really means.