This is story related in Half Life 2: Episode 2. At the start of chapter 2 ‘The Vortal Coil’ you come to an area with Alyx, a vortigaunt, Griggs and Sheckley. This area is accessed by four tunnels and antlions will attack from here. An alarm sounds when an attack is about to begin and lights indicate which tunnel the attack will come from. One light means the attack is weak, two lights means a moderate attack, and three lights mean a strong attack.
You have two turrets to help you and you can move these around to suit your preference. A turret can cope with a one light attack on its own, and Griggs and Sheckley can hold off a two light attack by themselves. I helped out with these though to protect the turrets from damage. Three lights means that you’re going to have to combine efforts and help out more. There are health packs and ammo crates lying around to give you a boost, and one of your colleagues is a medic.
After a few invasions, a team of vortigaunts appear. You will then face a three light attack from all four tunnels. The vortigaunts also help out here but it can still get a bit tense. The achievement pops when this invasion is over.