Can't get It's our Time Now Achievement

Ok, how the heck am I supposed to acquire this achievement? It's the only thing stopping me from 100%ing this game. But no matter how many times I have to defend the Homeworld from enemy attack, the achievement never pops up.

What do I need to do exactly? I have had my fleet on and off the planet (and sometimes my useless Megabots actually decide to show up to help out!) and nothing.