Just to simplify the excellent solution above:
Load up the troop screen. Now just under the name of the card you will see in slightly smaller writing the name of a city, this indicates the troop belongs to that particular city. Therefore every time you level up that particular card you will also increase that Kingdoms power. For Example, My Ancient Horror Troop has Karakoth written under its name, therefore everytime I level it up, Karakoth's power will increase.
On the troop screen you can filter down troops according to Kingdom. Press
on the troop screen, go to the KINGDOM DROP DOWN BOX, select the kingdom you want to increase power, then press
again. THis brings up the troops you have so far unlocked belonging to that city, and saves you having to look them up manually.
The very first power is Silver, but to unlock the achievement you need to get the Gold Star. Which requires 1200 points. Everytime you upgrade a troops level it adds 10 points onto the bar, each time you upgrade a trait adds 25 points and each time you unlock a new troop belonging to that city adds 50 points. Finally, to check on the progress, click on a Kingdom then select the screen for Levels & Power, press
until you reach Power, you will see a purple bar which indicates Power level, you need to fill the bar to 600 for each star.
the solution above is great and detailed but i thought I would simplify it for beginners as there is a lot to get your head around. hope it helps.