First off, it's much, much easier to do this with multiple people. I suggest getting it this week (Dec 3-9), while Episode 5 is in Matchmaking.
In the beginning of the mission, there are a couple Warthogs (Chain & Gauss). Take them and park them in front of the generators. There are also a couple more near the generators, so you can create a small blockade. Get into the gunner seats and shoot anything that moves.
There are multiple waves of varying difficulty (including Hunters and Elite Officers), coming from both drop pods and Phantoms; but with a little teamwork, this achievement should not be a problem.