I used the mace with a shield once.
Ewww that was slow.
But nevertheless, this weapon is totally OP. I can choose it with any character, and I'll always be capable of doing a lot of kills with it. I don't know why, but this weapon seems to ignore wpf entirely, in the sense that it's already fast as hell with 1 wpf.
To balance it, I think the best would be increasing it's dependency on wpf somehow and removing the knockdown. Knockdown in general is a retarded feature. Not only is it random based and extremely powerful. It also negates the stats of the victim (as in, a 30/9 mammoth can be knocked down by a 9/30 fairy without any problems, and the mace in particular is very popular among fairies). And doesn't require any adaptation from the opponent, since he simply has to avoid being hit, unlike crushthrough which introduces different threats than the usual ones, with different weaknesses.