I have not run into this per se.

I have had idiot reinforcements but I've always used them as a distraction.

Defensively I keep my troops in single large formations. While I might go to an allies aid I still bank on having to win the battle myself.

They are not always idiots but I don't let myself expect anything out of them to begin with.

Offensively I make sure I am hitting them from opposing sides.

This way I can usually count on my enemy getting distracted as I take them apart.