Context: a boy is always been a head shorter than a friend of his, and he always said:
which I think I can quite understand ("Someday, I'm sure I'll surpass him"), but not really, since the whole
抜いたるけんな baffles me.
I did some research, and I found that
けん can stand for
から, but I'm not sure this is the case (see later).
たる I found a couple of questions (this and this) which don't quite seem to apply, since in the first the structure is different (
stem-たる, while above is
short past-る), and in both is attributive, while above it doesn't seem so (unless it's attributive to
けん, in which case it doesn't mean
から and I don't understand it's meaning).
な I think it's just and ending particle, maybe for emphasis.