Chromehounds

Have your squad create a squadron of light weight scout mechs on either wheel or hover chasis. Then pick all piles for your weapons, bombs work ok too and if you want to stay alive and not lose are a bit better if you run across a defender mech but if you attack them you wont get any progress towards the achievement so avoid enemies at all costs. Make your way around the map and find your enemies base. After you find it, their flag will be on the side, Destroy it as quickly as possible avoid attacking your enemies at all costs if you really just want this achievement but you should be able to get it within a few hours with this strategy.