It is simple, make it 4 generators.....in order to kill the first generator you must kill the first wave of defenders, 2nd wave for the 2nd 3rd for the 3rd and so on...make the generators invincible until the corresponding wave is destroyed.
works in star trek online STF's to put down the zerg rush and would work here as well ...it would make it literally impossible to zerg rush and force people to actually think strategically...that is what this game claims right?...A thinking mans shooter....except if you do that..they would actually have to think lol...you are welcome