Boost is one way you can achieve. But their capability may still be too much for you. You need excellent defense. I have a moderate Attack/defense boost. 165/95. For example in the given below province managed to beat sector 1 and sector 6 with 8 rangers after infiltration. Even with boost when you face units that are 2 ages above you , it will be challenging as they have some special abilities. You have to wait until you get there or have much bigger boost like 700/500