One of my dictionaries defines this (actually, 一騎当千のつわもの) as
a matchless [an extremely mighty] warrior / a warrior who is strong enough to fight against as many as one thousand men.
My question is why was [当]{あ}てる used as the 「とう」 instead of [倒]{たお}す? Seems like it would have driven the point home a bit more.
(Of course I know speculation is off-topic, so etymological answers are what I'm looking for).