Well, as a fan I feel obliged now to beat the entire campaign on expert too, after finishing my current one on difficult.
But I'm stuck on the next map ahead of you, DURDERO where a vehicle of the enemy is stuck outside the visible map on turn 50 something. When you reach it, try to attack the main isle in the lower left corner first, even though that seems far more impossible. If you ever run across the bug where the game bugs with the AI repeatedly starting turn and ending it forever, this is an indicator that a vehicle moved outside the map.
So, if that bug (turn started.. turn ended.. turn started.. turn ended.. turn started... etc) ever happens, reload a savegame from the last turn and hope that he doesn't drive outside the map, otherwise you'll have problems to kill the last vehicle at the end.