There should be a random pvp quest that comes up once in a while called "enough with the bombs!" on the isle of thunder. This quest will spawn sunreaver magus around the alliance boats outside the hold. What i did to get the achieve is get the sunreaver magus closest to a ward and kited towards it. Might take you a couple of times to do because the alliance npc will tend to be aggroed and kill the magus when you are trying to get the achievement.