Pak 38s when cloaked are a viable counter too.
If you have a Marder being circle-strafed, does it help to position it on a road so that it turns faster? Or is the M8 just too fast?
The easist way to deal with a armoured car circleing you is driving backwards with the marder and blocking the armoured car with a kettenkrad, swimwagen or PGrenadiers.