The rebel on the other side will warn you about hoppers. These are annoying mines that can detect enemies and “hop” at them for more damage on explosion. Pull the metal sheet off of the hole and look down. Pull up each of the six mines with your gravity gun and then just drop them. An unnecessary explosion might just hurt you or your team. Pulling them up with the gravity gun will reset them to friendly status, so just let go of them. Do not shoot them into the air. Once these six are disabled, drop down and reset the other few in the yard. You might want to throw one at any surviving headcrabs on the upper level though. If you wait, the headcrabs will trigger the mines on the upper level.

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The rest of the headcrabs will probably be dead by rebel gunfire and hoppers by now, but if not, kill them and go through the office carefully and watch for the few hoppers on this level. Watch for one more hopper mine by the rebel who spoke to you at the beginning and then turn to the alley.
Go out this alley and just wait for a second to make sure you don't get caught up in any of the explosions in the street. There's nothing you can really do about the strider now, so just move up the street and use your AR2 or the Magnum to kill the one Combine soldier in the watchtower. Also try to shoot the scanners deploying the hopper mines. This will make it easier to know just where they are. You can also just use the gravity gun to catch the falling mines. This immediately resets them. Go up the street minding any dropped hoppers and turn down the alley. There are about two hopper mines in the little alley to the right. One is usually dropped into the truck too, so be careful. Get rid of them and move into the alley. Knock the furniture out of the way and enter the lawn series.