Under cover of dark, out of view of other players. Perhaps use an alternate account that has a different avatar and displayed name. You can use your basic pickaxe/hammer/chisel to quickly do significant damage to most structures- but it's important not to linger. As in all FPS games, a stationary target is a soon dead target. While you're attacking your enemy's structure, watch out for native creepers passing by, be ready to move out and not leave anything behind at a moment's notice. Ideally you can have a guild, and a team member who will be using their normal account watching from a distance- they can PM you if they see danger and alert you, and assist in the getaway. I wouldn't suggest trying to fortify your own place on their base, because it's their home territory- they know it better than you and have rights to it. If an admin comes in and finds you griefing them, they may boot you or even worse. Move in quickly, quietly, have all of your tools easily accessed on your hotbar or otherwise somehow fastened to your character (you don't want to leave ANYTHING behind) so you can quickly make your changes, then bounce like Mario. You might want to quickly change accounts, so that if an admin shows up and checks your inventory, they won't see anything that could be connected to the griefing. You really shouldn't treat people like that though, man. Minecraft is just a game, and a lot of kids play it, so doing something like this to something they built might upset them and that would be mean.