TL;DRMake it so player can produce combat units that he can control like in RTS games.
What ?Here is how it would work: Player builds a building that he can produce units from like in RTS games, let's call it Soldier Factory. Then he gives those units simple commands like attack here, move there, patrol from here to here etc. just like in RTS games. There could be a couple different types of those units and buildings producing them. Also player should be limited in how many of those units he can make by the existence of another building that is required to control those units, we could call it AI control center. And there could be another building that would limit the area in which those units can operate we could call it AI control tower, so you can't just walk with them to the end of the map. And a couple of unit ideas that I think would fit nicely:
AI Soldier: Small units that just walks around and shoots things with machine gun doing not too big damage, have little HP. Best used in swarms against not so tough enemies so the enemies die before they can reach soldiers. Inspired by marine from Starcraft.
AI Flamecaster: Also a small unit but with a bit of armor so it can last a little bit longer if aliens get to it. Uses short range flame thrower that does AOE damage. Inspired by Firebat from Starcraft.
AI Dropship: Can be loaded with other units to move them around the map swiftly so they don't run into trains or have hard time pathing around buildings, can cross lakes and land on islands if you play with lot of water. Inspired by dropships from Starcraft.
AI Tank: Tough big unit that can take a decent amount of damage. Have two modes of operation: 1 - it is mobile, does a lot of damage to a single target at a medium range, 2 - gets immobile, does a big splash damage at a long range. Inspired by Siege Tank from Starcraft.