It helps a lot if you have hitched up the flak gun from the previous mission to your truck (the game tells you that you can bring through a towed gun as well as whatever you get in the truck).
I cleared all the enemies outside, then the south gate, then up to the main building with thrown knives since I fancied being stealthy first.
Then I lobbed down AP and AT mines as far out from the building as I could without getting seen by the enemies left alive and brought in the flak gun and the machine gun from the south gate and popped them on the lawn.
Next I capped the admiral and cleared the buildings to the north with the initial reinforcements and set up guys around the perimeter wall covering from 3 o'clock past 6 o'clock to 9 o'clock, keeping the AT rifle just behind the line.
All the light vehicles fell easily to MG fire and mines, only issue was the tanks but the flak gun can drop them at the range they stop at to fire... also bring forward the AT rifles to fire when any vehicle is getting close.
If you have a save from the previous mission just grab the flak gun, hitch it up and it's not that hard to defend for tyhe negotiations.
Lastly... all your reinforcements should be men not the ammo truck.... the poxy truck brings mines you can't lay cause you are under attack and less total ammo than the men turn up with

You can scavenge ammo off dead germans anyway.
Hope that helps.