Steel-man is such a weird expression. There are no steel men. How about saying "The opponent's best argument".

The steel men (armored enemy knights) are exactly the inverse of the straw man (training dummy) metaphor. I think it's a fantastic term since it directly addresses the point (tackle the best opposing arguments head on instead of a poor subset/facsimile of them), it fits within the existing straw man metaphor, it's terse, and it's very clear.