I used the 4th mission on easy. I would instantly buy 2 Rocket power-ups from the in-mission store (you get 6, 3 costs 15,000 points) I would then wait for enemies to clump up a bit and let it loose. After finishing the game it took me 3 more plays of the 4th mission. Sometimes I would have to buy one more, it won't take long as you will be using a lot of these to take out buildings and tanks in the campaign.
Price updated thanks to Dragon Weew
It has also been pointed out that if you kite the enemies a bit you only have to buy one power-up, which can save command points and is more important with the updated power-up pricing.