this is mindbogglingly simple and easy. all that has to be done is get a lot of lava. water+lava= obsidian, so all you have to do is make a lava trench. only one block deep, or it dosnt work. pour water in, and you get obsidian, and this gets you a obsidian fort fast. the best way to defend against the resistant is the command block. just command it to kill anyone that is nearby, and you're set. also, make heavily armored bedrooms. 10 block thick obsidian kills TNT blasts at their source, and a wall of lava o'er the obsidian stops the diamond pickaxe. if it is an op that's the problem, get them banned.