Try to keep your available resources below your 'safe' amount so that you are not a target. Having a mix is good. Rowdys are a cheap way to hold off a lot of damage from more expensive units. Bouncers also help hold off damage, and they do more damage back. Big Mama's are good for defense. Bazooka guy's are good for doing lots of damage. Mercenaries are good to add once you have enough of the smaller ones, as they are a good final line of defense.