Requirements--you do NOT have to kill capturing tanks, you just have to hit enough tanks capturing the flag that between them have a total of 70 capture points, in one game. A hit on an enemy tank resets its counter for capturing. Wait until the capturing 'seconds' or count is at at least 70 before shooting the invading enemy tanks to reset their counters-- You do not have to wait until the capture count is at 70, but it's recommended to wait until then to get your best chance, otherwise enemy tanks might spot and kill you too quickly.
Also, the achievement pops at the end of match screen as it is tied to earning 3 of the "Defender" medals.
I think any Tier 1 tank on the map Province should work, but what worked for me specifically was:
Chinese Tank Tier 1: NC-31 and the map Province - Night.
It's better if you spawn on the west side of the map, because there is a house on the south edge of the map that is perfect to hide behind. Go straight there, hide right up against the house, and hope your team dies fast!
Once my team finally almost all died, the other team had two tanks capturing our flag. I drove out from behind, shot one tank to pieces, then hid again. Once my shots reloaded, I did it again and got the flag clear and 1/3rd of the achievement!
US Tank Tier 1: T1 and the map Mines - Rain.
I spawned on the south side of the map where at the edge of the map a road goes between two mountains behind the flag. I went and hid behind the building that has a large rock blocking the view towards the enemy's base, and just waited. Happily, the other team started killing my team, then 2 of them started capturing my base! I stuck my head out, and started shooting them. I almost failed though because I missed a lot of shots I was in such a hurry, but after 2 reloads I was able to kill them both and get another 1/3rd of the achievement!
after about 3-4 other matches, either 15 player or my team winning, this one happened:
Chinese Tank Tier 1: NC-31 and the map Mines - Rain.
Once again I spawned on the south side of the map. I went and hid behind the same building and waited. The other team did much better this time and got 3 of them capturing my flag after a little while. I rolled out and made sure to hit as many of them as I could because I thought they would kill me. Sure enough, I got killed quickly, but I was able to spray my bullets over all of them and knock their capture counter down to 0! I the final 3rd of the achievement, and the achievement popped on the match results screen!
I recorded a couple videos of the camping, somewhat boring, but if you wanted an idea visually of where I was camping on the map Province plus some action:
And this is where I was camping on the map Mines: