1. Ram the hunters with your car. They'll dodge out of the way if you do it head-on, so rush them at an off-angle, then at the last second steer into them.
2. Supply houses automatically restock after you've been away from them for long enough. Don't worry about trying to get the "Neighborhood Watch" achievement; it's okay if you lose the sawmill early on, but try to keep the rest of the buildings intact until near the end.
3. Never leave a supply house without a Magnusson Device (Strider Buster) on your car's back rack.
4. Catch hunter flechettes with any physics object (usually a log, they're pretty common on this map), and then punt the object back at the hunter with the gravity gun to instantly kill it.
5. You can also instantly kill a hunter with the alternate fire on the AR2 (the energy ball). Well-aimed revolver shots are also highly effective against hunters, but to save ammo, try using logs or other physics objects whenever you can.
6. After all (usually only 2) of a Strider's hunters are dead, then get your gravity gun out (with the G key), grab the MD, and punt it at the main body of the strider.
7. Memorize the layout of the valley, where the buildings/caches are, and what direction you're facing. The HUD showing your car's location will help. Someone else on Steam created a guide that shows when and where each strider spawns from, what it does, and where it goes. You can use this to your advantage. https://steamproxy.com/sharedfiles/filedetails/?id=171147837 This gives more details about which striders attack which buildings and when. If you're not trying for the Neighborhood Watch achievement, then my ten tips here should be sufficient enough for a basic defense, especially on Easy difficulty.
8. In addition to one MD/strider buster on your car, you might also consider hiding another strider buster in a nearby location, such as behind a train car, or in one of the buildings, so that you can have three or more MDs in play at any given area: Your car, the dispenser, and the "hidden" spot. You can set this up before your first approach to the sawmill, when there isn't any battle happening.
9. Other effective weapons for dealing with hunters include the SMG's alternate fire (grenade bomb), and a full shotgun unloading, 2 at a time (3 double shots), to the hunter's face.
10. Lastly, don't worry about losing the Sawmill, or a few of the other buildings later into the battle. During the final wave (when the music starts), you may be able to get by with fewer buildings, fighting from closer to the base. There's a final MD dispenser near there which you can use on the last few striders as the rest of the resistance members shoot at the hunters and striders, including shooting rockets at the striders to stumble them for you.

(Image borrowed from CombineOverwiki.net )