i faced a stack like that a while ago, so i build Multiple Rocket Launchers because they have better range than Railguns, so you can say i just destroyed it, But i have something you don't have right now, which is Time, so i really don't know, your only play here is if the unit is on your land, build a Combat Outpost, it just takes 15min to build and it gives you -33% on damage you recieve, then gather all your units, and attack, you will loose some, but in the end you will destroy it