On the option menu, you can set the "No Heat", "Invulnerability", and "Unlimited Bullets" options and still go forward in missions. Note: This doesn't apply the your Lancemates, except for "Unlimited Ammo" and "No Heat".
Strip a Novacat of everything but armor, including heat sinks and components. Fill it up with as many large lasers as possible, and one medium laser. A suggested grouping is Half and the medium on 1, Half on 2, put all of them 3. Make sure to add any extra tonnage to armor and speed. Change the armor to Ferro Fibrous. This setup is incredibly powerful and can bring even the strongest mechs down in just two or three hits to the torso.