Guyra108,289 Locked 28 Jun 2021 28 Jun 2021
In chapter 4: "Urban Flight", you will kill a sniper, and Alyx will climb into the sniper's nest. She will any enemy near you as you continue on. So let her take care of any enemies that appear, and just run around for a bit.
In the first area there are zombies and headcrabs, as well as one soldier in a makeshift tower. Use your gravity gun to take him out before he kills too many of the zombies, as they are easy kills for Alyx being so close.
In the second area you'll face continuous soldiers and antlions. The antlions are a bit harder for Alyx to kill due to their speed, but you can use your gravity gun to temporarily stun them. Since both these enemies keep spawning until you block the holes or enter the gate, you don't have to worry about accidentally killing some.
Just run around without advancing through the armored gate until Alyx has killed 30 enemies, and the achievement should pop!