They should not be able to destroy the mex that is the farthest away, which you should not destroy. A possible strategy is to build T2 artilleries at the down left mex and let them shoot at the factories. Make sure to upgrade your economy to Tech 3, you will need it when you complete this mission.
If the enemy sends engineers, you can capture them to gain access to Seraphim units, and this is especially important for UEF because UEF lacks a tough land experimental as well as an air experimental, and the ability to produce Ythotha and Ahwassa is invaluable. The enemy will send small waves of harassment units while you build up a base. Your first mission involves destroying a small base of a Seraphim factories to your west. This starts out as a fairly basic mission, but there is a twist. There is a substantial amount of reclaimable mass in the ruins of the base surrounding your start location, so you can use them to boost your early mass generation. You start with nothing but your ACU, so a standard Build_Orders beginning is necessary.
