Why can't take these arty units ?

[NicolasBourbaki]

Hi, 

I've been trying to destroy these 2 Prussian arty units but without success. I've launch many melee, fired many times and nothing will work ! All my units end up routed, including large bataillons of 500+ men ! All this for 8 + 4 poor guns without any Prussian infantry unit defending the hex. Just the guns. 

This is from "Rep. Bayonets on the Rhine" game. 

Any idea/explanation ?   

Thanks

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