After a lot of losses and a few wins, here is the setup my squad uses to hold a sector against much bigger pushes.
1. Two Breachers on the main chokepoint, shields overlapping. 2. One Heavy Gunner on elevated cover covering the approach. 3. A Marksman watching the flank everyone forgets. 4. A Medic behind the Breachers, never in the open. 5. A Drone Operator spotting the next wave before it arrives.
The secret is the rally point. Put it somewhere the enemy has to walk past your gunner to reach.