The first group won't be attacked, but enemies will spawn while you escort every group after them, allowing you to learn the path. Be sure to take note of how many survivors are in each group (there's 5 groups total, with member counts of 4, 2, 4, 2, 2). Alyx will stay behind to man the standing turret at the entrance to the station, allowing you to lure some enemies in and have her kill them.
It's also good to just sprint across the initial section as the enemies spawn in. They might try to pursue you, but the single entrance makes it easy to take them all out at once. You can also use the squad command keybind to summon them all to you.
You might need to quicksave/load a few times, and I'd recommend quicksaving inside each building entrance just in case.
Clear out all the enemies after a group makes it to the station (that way the next one doesn't get hit immediately). Seems like enemies only spawn when there's a group being moved.
There's also a rocket launcher ammo box near the first entrance (just behind the dumpster). It's fairly hard to miss, and can be used to easily kill any enemies nearby (and on roofs). There's also an HEV supply crate inside the dumpster you can destroy to get a bit of extra health or suit charge.
Other than that, this is really just a test of your FPS skills. Make sure to use all your weapons, as this is the final chapter (and you've got near-infinite ammo for the SMG and Shotgun since enemies drop them).
Barney will join you in the final group and help kill and Combine (though all the groups also try). If done correctly, you'll get the achievement after the last group reaches the station just before the conversation triggers.
I almost got this alongside The One Free Bullet, but I do advise against trying to get them both at the same time. Can be frustrating due to RNG.