Nice try but it is nothing to spoil. This is another facet of the impervious condition. If you only attack monsters that you would ordinarily attack nobody will think twice about it. Attacking monsters that are usually beyond your abilities is frowned upon by some. It has never been officially, specifically, sanctioned or banned as far as I know but it is technically abusing a bug.
That said if you want to train weapons you will end up doing it as it is a pretty common occurrence. Note, though, that your armor skills will get no practice unless the opponent fights back.
