I was wondering about this and came up with a possible solution. While they look and appear to be menacing and scary, what if they are liken unto a boggart and this so-called 'powerful' Dark Wizard is merely manifesting something that he thinks is scary but in actuality the menace is nothing valid.
So, what if when the sandmen appear (and the wizard is around) you just looked at them and laughed? Sort of like '
Riddikulus!' and imagined them to be gorgeous sand sculptures - a whale, a castle, a merperson, a thestral (etc). The wizard would most likely get upset and lose his concentration and they would fall apart. Who knows, they might actually turn into the sand sculptures, too.
