Strategy[edit]
Option 1: Stay put and wear the enemy down with artillery fire while targeting enemy aircraft and AA emplacements. Charge south when the enemy’s forces have been weakened.
Option 2: Withdraw immediately to the small area around your HQ, and abandon your Depot and forward bunker. Let your infantry take up positions on the ridges, put your artillery on the road behind them, allow the enemy to close in and wear him down with indirect fire from your artillery and bunkers. Enemy AA emplacements are priority targets. Do not otherwise attack with infantry yourself. Move damaged units back to the HQ for repairs and replace them with fresh reserves from the rear in order to keep holding the ridges. Note that when the Depot is lost, your repair rate will drop. Keep your aircraft on the airfield and engage the enemy aircraft there, where they will be within range of your AA batteries. When the enemy is worn down sufficiently, break out and recapture your Depot. Finally, close in on the enemy Depot and HQ.
(edit): A variant of option 1 is to be preferred. Just fall back one hex with your whole line in turn 2 and a few turns later, to stay out of range of the enemy's medium artillery. Also, it is extremely important to take good care of your planes. Since you have no factory and can't build new ones, don't lose them under any circumstances. Without them, the endgame (hunting down the enemy heavy and medium artillery) will be much more costly (and tedious).