This achievement requires that you successfully invade friends' kingdoms 100 times. This is the easiest of the online battling achievements. However, you do need to have Xbox Live friends that have the game to be able to invade them. Head to the Social screen (under Tools) to see all your friends with the game. Here you can select each friend and choose to invade them. Once in their kingdom, click on any building to invade it. Win the battle to get credit towards this achievement. You can only battle each friend once every 24 hours. So ideally, the more friends you have, the quicker this will go. Your friends do not need to actively be playing the game. They just need to have started a world and created a cloud save at some point. You can check your progress towards this achievement under Tools>Achievements (there is a counter).
See "Demon's Nightmare" for a good method to boost that achievement and this achievement simultaneously.
TIME GLITCH METHOD: You can exploit your device's time settings to earn this achievement quicker. As mentioned, you usually have to wait 24 hours before attacking the same friend again. However, you can exit the game, set your date BACK one day, load and exit the game again, then set your date back to automatic, and finally load back the game. This will allow you to attack your friends again. Basically you can just attack all your friends and then use the time exploit to be able to attack them again immediately.