If you're worried about getting City Defender on a PvP server I wouldn't worry about it too much. I just finally transferred back to a PvP server and I acquired that achievement within the first week or two. The Horde are always attacking our capital cities and vice versa. How long would that take to keep running back to your corpse so your alt could kill it over and over again anyway? I would think the rez timer would be up to 3 days by the time you got 50 kills.