I'm going to write a solution for this, but I'm afraid it won't be fast or easy! I'm simply writing this to hopefully give a few tips that might help. I had a very hard time getting this achievement, it was my second to last achievement for the game!
The problem is that your team usually either kills the enemies before you finish capturing, a teammate jumps on the flag with you and captures 21+ points(you must get 80 out of 100 points), an enemy damages you and restarts the counter, etc. I have only 7 Invader medals over nearly 4,000 games(The previous 4 medals didn't count when I started the Xbox One version). Now, I didn't try for them every game, but that does say something!
Some possibly helpful ideas:
1. Wait for the right opportunity while going for other achievements!! I can't emphasize this enough. Since this is extremely luck based and requires MANY factors to go your way perfectly, just wait for the opportunity and don't try to do it faster than it naturally happens.
2. Look for opportunities! If there are a few tanks left on both teams, and the other team has a heavy/arty/tank destroyer, you might be able to make a base capture.
3. Tier 1 is possible, but only on certain maps. Mines is a good option, Province is pretty much NEVER EVER going to work unless the enemy is AFK.
4. The longer you survive in your matches, the better chance of getting this achievement, so, figure on getting this with Heavy/Medium/TD's. Light tanks and Artillery just aren't built for this, although you might get a very very random match where it actually works.
5. If it comes down to a 1v1, try to figure out where the enemy tank is. If it's a slow tank, then you have a very good chance of doing this, but pick your capture location very carefully. My last Invader medal happened when I was 1v1 in a Tier 6 Chinese Medium vs a British Tier 6 AT6. He didn't have enough time to get back to their base... almost. He had time to get off one shot but I was hiding behind a destroyed tank. LOL. Alternatively, tracking Tank Destroyers can help you capture. If they can't shoot you because they can't turn, then you're safe.. just don't kill them.
Another options that I didn't have available is possibly platooning this. Do it on a Tier 1 match, have your teammates wipe the map after letting your random teammates die, then have them either track the last enemy, or keep him busy while you capture. Still, quite luck based, but slightly better chances!
If anyone has any tips, please add them in the comments!! This was a pain to get.