So I've gotten every Heroic victory in the campaign till this map. I'm finding it impossible to do, so I wanted to see if anyone's got any tips to achieving it. I've tried several strategies but so far only a lucky map has gotten me to what I felt was close to the Heroic victory; there was only one access to my gristmill so the chokepoint allowed me to hold for relatively long.

I'm close to being frustrated since I've tried it at least a dozen times, so any help would be welcome!