This achievement requires that you drive an occupying enemy out of your kingdom. When another player successfully attacks a building in your kingdom, a symbol will appear that you can click on to battle that player to drive them from your kingdom. You need to do this 100 times. You can also help a friend resist an invasion by clicking on the same symbol when you are visiting their kingdom. Win the battle for them and it will also count towards this achievement. You can check your progress towards this achievement under Tools>Achievements (there is a counter).
There is a good method to boost this achievement, as well as the "Demon" achievements simultaneously. Basically you want to recruit as many friends to boost with as you can. Each player should keep five, low-level infantry units as their only line of defense (either spear men, axe men, or sword men). That way, when your friends attack you, they can deploy five low-level enemies that has a damage bonus against your defending soldiers. They should have no trouble successfully invading your kingdom. Now, when you click on the symbol to resist the invasion, you will only be fighting their low-level enemies. You can pick whatever powerful units to rid them from your kingdom. This assures that everyone will be able to get easy invasions and defends against the boosting partners to work towards both of those achievements at the same time. This method is especially effective because it eliminates the issues caused by players having different levels. Each player can only attack you once per 24 hour period (without the time glitch), so to get this done in a timely fashion you will want as many friends contributing as possible. See the Achievement Trading Thread for others looking to boost.
TIME GLITCH METHOD: Basically you want to visit a friend that has been attacked by another of your friends (must be friends with both). When you visit their kingdom, the symbol will be there to repel the attack. Then click the icon and win the battle as normal. This will count towards the achievement. Then exit the game. Go into device settings and change your date BACK one day. Load game and visit that kingdom again. You should be able to repel the same attack again. It will count towards the achievement as well. Then exit the game and return time settings to automatic. Load and that same repel should be available again. Repeat over and over. It is kind of tedious, but is better than waiting around for friends to attack you. If you can find multiple friends kingdoms with repels in them, then you can do more than one before you have to change the time settings and reload the game, which saves significant time.