City Defender

Most of this has been said, but I'll consolidate it into one post.

What you DO have to do:

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• Deal damage. The only way around this requirement is if you're in a party/raid and one of your members gets the kill, in which case you will get credit even if you did no damage.
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• Be inside your city. You will get no credit towards this achievement if you yourself are not inside the city, even if you kill a target that is.
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What you DO NOT have to do:

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• Get an honorable kill. Any player of the opposite faction, regardless of level, will count towards this achievement.
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• Land the killing blow. As long as you do any amount of damage to the target or are in a party/raid with someone who gets the kill, you will earn credit towards this achievement.
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• Kill a target physically in your city. As long as you yourself are inside of the city, killing a target will contribute to this achievement, even if the target is outside of the city.
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Friend of mine and myself just stood on the Stormwind <--> Borean Tundra boat and I killed him everytime the boat got into Stormwind Harbor for my achievement, and he'd just kill me when we got to Northrend for a HK. Since the boats auto resurrect you upon zoning, he (being horde) didn't have to take the 10 minute spirit walk from Eastvale Logging Camp.

Also, for some reason I was getting credit for 2 kills each time I killed him in this fashion so achievement went by even faster.

Easy way to get this achievement for Alliance players is during many of the World Events, the one in particular is Pilgrim's Bounty.

During Pilgrim's Bounty just camp any and all Horde players that come to the Darnassus tables, as they'll have to have gone through Darnassus thus pulling some guards (unless they hugged the north), and will be PvP flagged. Plus the tables are well inside Darnassus, so even if they run you'll stand a fair chance at killing them before you change subzones.