魔王は言った。「俺が負けるわっきゃねーべ。審判は皆地獄にいるのだ」。

。「。」。

The Demon King said: 'There's no way I'd lose. All the referees are in hell.'

Literal

Demon-King [topic-は] said. 'I [subject-が] lose no-way. Referees [topic-は] all hell [in-に] exist [explanatory-のだ].'

This sentence is rich with dialectal slang. わっきゃねーべ is an extreme contraction of わけがないだろう ('there's no way') — collapsing わけ→わっきゃ, ない→ねー, and adding the dialectal copula べ (northern Japanese). 俺 is the rough masculine 'I.' The joke: the Devil can't lose because all the referees are in hell (his home turf).