Select the 60 ton model and go to the customization screen, then strip away the large lasers and replace them with medium lasers. Go to the LRM-20s and increase the ammunition up to three tons each (eighteen shots total). When you attack, use a slow approach. Get a lock with 2x LRM20s, and hit them from far away. Almost all of the time they will go down in one salvo. An Atlas might require two salvos. With eighteen salvos at your disposal, you should be able to finish off every enemy in the mission without using the lasers.