It's a bit difficult to work this mission out but here's how I did it...
You get the "G.....URRRRRRRRRR" radio call if the two hinds take out too much of your ground element. To complete the mission, I gave the wingman the complete mission goals command. I think that was it. As he was engaging the convoy, I took out the hinds.
The enemy's convoy and the hinds have to be taken out, without too much of the allied convoy being destroyed, for the mission to be completed.