This can be done at any level or city, but I would recommend that you do it early on in your city development of Kimberly before you have fortified your generator. This will limit the amount of cost you need to recoup with removing defense systems or extra generators.
The first thing you want to do is move all of your offensive weapons to one side of the city and place your only generator alone by itself on the outer-most ring possible. On the outside ring, place three of your Bulwark E shields. Remove (demolish) your Counter Missile B defense system. Try to chose a mission, preferably a level 1, that has the Antrion (the squid-looking enemy). Now, as long as the enemy is not within that red-highlighted region it will be able to remain free from attack and you can allow it to destroy the generator.
You want to wait until you have reached about 50% (60%-75% is ideal) or more of the mission completed before changing up your battle tactics. You need to have your generator attacked until it has been "destroyed" (it doesn't get destroyed if you do this right) and a 2:30 timer will start. Try to have the Antrion be the only enemy on the screen so it will be using it's long-range attack beam. Shift your city so your generator is the only thing on that side, place your Bulwark Wall to the left side of the generator and be ready to intercept the beam weapon when it shifts from the generator to the Command Center. This will limit damage done to your city. Just watch the green heath bar and wait for it to be reduced to 0. To get the trophy you must finish the mission before the timer expires so you want to make sure you have enough firepower to quickly eliminate the remaining assaulting creatures. Once the timer starts, it's all hands on deck and I'd recommend using your long-range Missile weapons as the primary force to remove enemies as soon as they appear. I had my missile system set up as a pair of triple-aligned weapons (total of 6), all improved up to Level 6, and finished off the round with about 30 seconds remaining when starting the counter at about 55% complete on the Easy difficulty setting.