Mission 13 in the Axis campaign is your final counterattack bonus level, returning to North West Africa for an additional 20 waves of enemy aircraft and ground units. Aircraft will show up early this time around at wave 2, and the surprise attack will severely damage a large amount of your turrets instead of opening up a new ground path.
Placement of weaponry will mostly be focused on building up around the previously established fie-bases. Unfortunately, due to the surprise attack, the starting funds you are provided will likely be consumed in repairs. You can sell any turrets you don’t have the funds to repair and are likely to be destroyed to avoid losing those funds.
Your main focus can be centered around the X-Weapon right off the bat, considering the substantial defenses you should be starting out with. Make sure to place some additional AA when you have the free funds, or when demanded by more challenging waves, as this is the only level you can be attacked from two paths of air units at the same time.
Machine Guns still serve their purpose as fodder for the enemy’s guns when placed strategically. Following the overpowering weaponry strategy, it should be possible to get 6 fully upgraded X-Weapons by wave 20. There really isn’t much complicated beyond the extra air defense for this mission.