Context: a boy tells to a trainer that he wants to become a boxer, so the trainer starts testing the boy's skills. After seeing that he's strong, the trainer says "なっちゃねェな". I think it is a contraction of "なんてことはない", but even so, I don't understand the meaning in this context. Thank you for your help!
It is actually a contraction of 「なって（い）ない」 which means "to be no good". So in your situation, the trainer is saying that the boy's boxing is no good. Note that is used to say "no good" as in "someone is not good at something", and not that "someone himself is a good-for-nothing".
On a side note, 「なんてことはない」 in the same speech pattern as the trainer would be something like「なんてこっちゃねぇ」 or 「なんてこたぁねぇ」.
There are a couple of things it could be a contraction of.
になっちゃ（う）ね ＞＞ になっちまう which is a contraction itself of になってしまう.
The ちゃ is a contraction of ては usually like じゃ from では. You also have なきゃ in the same vein coming from なくては. ～ては（いける・いけない） and ～てしまう are common phrases so can be shortened. してはいけない、しなくてはいけない、してしまう become しちゃ、しなきゃ、しちゃう etc.