Home Guard II There is no easy way to unlock this achievement, to get this you need a camp built, I recommend that you upgrade it fully as soon as possible because the more units it can hold the faster this will go.
2nd, you want people to attack the troops that come out of it when they attack you so place it somewhere people will hit first when attacking. Once you get attacked and its empty, reload the camp and wait (people will attack you when you are not logged in).
Once you refill the camp with 500 cumulated units this achievement will unlock. A good thing to do is bait people, max your base level and max your green crates (primary currency), the more crates the more people will attack. Also make sure to fill your base with JUST infantry.
Before going for this achievement I recommend having this one first:
Losing battles will take away your dog tags, you need to have 2500 at once for that achievement.