Strategy to counter the roaming zerg?
First: Zergs are lazy. Few people in a zerg ever “look behind them”. Sun Tzu actually address this. The short version is:
“When facing superior numbers, divide and conquer”
Zergs are easy to pick apart. 2 FGs flanking a zerg will tend to pull 2/3rd one direction and 1/3rd another direction. By staying at the peripheral of the zerg you can peel a zerg like an onion very quickly.
A good solid guild run zerg will be harder to peel but being in a zerg how often to you hear people spam: “DON’T CHASE”. There is a reason for that, chasing is peeling the onion.
don’t hit them head on. Supplies are most important. get them to split their forces into managable groups. Hit their supply lines randomly , also harrass their tower doors feigning an attack. Once they start to respond, move away and start somewhere else. put enough crosses on their map they will split zerg to defend. Wipe out smaller groups as you move.