How to raid claimed land in Minecraft servers?

It depends. Ask an admin if "Pillage' or "Plunder" is set to true. If it is not, this will not work. Once a faction is enemy'd you should be able to attack faction members in their own territory. After the faction's members die a certain amount of times (you must be patient, it will take a while) their faction power will decrease, and they will slowly start to lose their land the lower their power gets. Once it gets to zero, they willunclaim all of their land and it will be free for all.