Wrath of the Alliance

This achievement is extremely easy if you can find a friend to help or simply own two accounts.

1. It doesn't have to be Honorable Kill. Level 70 killing Level 1 counts
2. You can repeatedly kill the same enemy player to get credit

I two-boxed it and got the achievement in about an hour. It's much easier to create a new character near the corresponding city (e.g. undead player near Undercity) than walking the level 1 to another city.

I'm Alliance so Undercity and Thunder Bluff are very easy to do. There's no guards in the frond entrance of Undercity. In Thunder Bluff, you can simply kill the level 1 under the "hills" and it still counts as credit. Orgrimmar and Silvermoon are much harder because of the guards.

It seems like both your character and the target enemy player have to be inside the city boundry to get credit, but I'm not 100% sure about this.

Do these have to be honorable kills?

This can actually be done with one account, and two computers.

Put both your toons in the same place, so you are marked as being in the city. Log in with your horde toon on one computer, flag yourself for PVP. Then log into your account on the second computer with your alliance toon. You will be disconnected from the first computer, but when your ally toon arrives, your horde toon will still be there. Kill, rinse, repeat.