だけど勝{か}つのは何時{いつ}も金太郎で大{おお}きな体{からだ}の熊{くま}さんでも金太郎には勝{か}てません
So let's start from the beginning:
だけど
introduces a contrast with the previous sentence similar to but
or although
.
勝つのは
nominalizes 勝つ
and introduces it as the subject using particle は
, thus the one who wins is
何時も
adverb meaning always
金太郎で
: This is the part stating that it's Kintarô who wins. で
is the continous form (連用形) of the copula だ, here indicating that the sentence isn't over yet.
大きな体の熊さん
: "Mr." Bear's big body
でも
is not the conjunction でも=but, yet, still
, but the adverbial particle (副助詞) でも=even
. See these questions for more details: 1, 2, 3 If it helps, think of it as 熊 で+(あって)+も
, even if it is a bear
勝てません
is the potential negative form (=cannot) of 勝つ
.
So I would conclude by saying:
Although (something), the one who victory is always Kintarô, (and) even Mr. Bear with his huge body cannot win against Kintarō.
Or a bit further from the Japanese:
But Kintarô would always win, even the Bear with his huge strength was no match for him.