To sum it up, it's a one-on-one showdown, but the level of play is beyond what ordinary people can imagine.
Literal
In-short, one-on-one match [explanation-なんだけど], that match content [topic-は] ordinary-person [by-では] can't-imagine [extent-ほどの] high-level.
タイマン is slang for 'one-on-one fight,' often with a street-fighting or delinquent connotation, despite the Chinese-character-like spelling タイ. Note ほど ('extent') with の making it an attributive: '考えられないほどの高レベル' = 'a high level to the extent that you can't imagine it.' 常人 ('ordinary person') is a slightly literary noun. The で in 常人では is 'by / for an ordinary person' (agent of the potential). 勝負 has broader meaning than just 'match' — 'contest, showdown, decision.'