There's no easy way to grind this out, it'll go slower than grinding scout or soldier. Here's the problem: When you're grinding defender you have to be within certain range of a combass, a headquarters or a team mate. The only thing that my random testing has been able to come up with is to snipe the a.i. target as it's taking a tower. It's got to be a clean one hit kill though. The moment that the a.i. figures out that it's getting hit, it runs and clears away from anything that will get you defender points. I'm not saying that it can't be done, but it's a pain in the rear to do it.