I got this while trying for this achievement:
The Defender achievement in World of Tanks worth 157 pointsReduce the amount of enemy capture points on a friendly base by 70 or more, 3 times. MP only.
It's worth just combining both achievements as you'll most likely get this one along the way!
You simply need to wait until an enemy tank starts capturing your base, then kill it, and do that twice in the same match. *Neutral flags work too, thanks Hirsute Dave!*
Here's what I did to get this achievement:
I think any Tier 1 tank on the map Province or Mines should work, but what worked for me specifically was:
Chinese Tank Tier 1: NC-31 and the map Province just got me the achievement. 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 the achievement instantly!
I also did the same thing later on Mines while going for the 3 Defender medals. The south-east corner of mines has a building near the mountain, and a giant rock attached to the mountain right next to the building. I just hid behind that rock and hoped the other team would take out my team. It took a few tries, but when several were capturing my base I jumped out and killed them both.