This is not quite an entire new solution but a little tip that helped me finally get it.
Do not be afraid to die.
I explain myself:
After getting just about every other achievement in the game I was left with 4, two of them are just a matter of time, Captain and Lt Colonel purchase, one was a matter of skill, skunked, and the last one seems to be a matter of pure dumb luck, If they came to hear me beg.
I've been playing invasion every night for the last 2 weeks or so with very frustrating results because as many have pointed out some people will vote for invasion slayer or not have commo or your team will suck etc. After deciding I would never get it I took one last chance at it last night and, lo and behold, it popped.
So finally for the tip, DO NOT WORRY ABOUT DYING, but make your deaths count, when you are defending use your sticky grenades at point blank range, even if it means you die with the enemy; you will spawn close to the defending point while they go back all the way to the river (Spire). That's how I got mine, granted I had a good team in place finally but it was me dying while sticking the enemy about four times that kept my undermanned defending point safe because everybody else save one other guy was tending to the other point.
Sorry, long rant about something so simple but it helped me, hope it helps you too.