I don't remember this. If you're talking about the bit in OotP that was an unprovoked attack if I remember correctly.

Slughorn was never shown to be a duellist. Knowledge of the Dark Arts is not proportional to your skill in duelling, as Harry himself demonstrates.

I concede to your Shacklebolt statement, while pointing out that the Death Eaters Dumbledore absolutely destroyed were no pushovers either. I don't remember, but I think Dolohov, Malfoy, the Lestranges etc. were more than a match for the best of the Ministry.

The only answer which makes sense here is Harry's sacrifice, imo.