First build a War Factory, a Barracks, and a Tech Center. Biuld several IFVs and the same amount of spies. Put the spys into the IFVs. The IFVs will act the same as if you put GIs in them. Send the IFVs to the enemy base and get one close to the building you want to take over. Unload the spy and he will run into the building without taking damage.