Live Bait and Citizien Escort below. Video by Axnollouse.
Or if you don't want to watch:Load the first level you meet up with Alyx, and explore the city. As soon as she gets sniper rifle, a lot of antilions/combine soldier and a few zombies will come up. Don't worry about kill count, you can kill everyone, because in later section of this level there's infinite ammount of antillions, so you can get it in no time.