This achievement/feat of strength is not meant to be obtainable ingame as the endless mode required was removed before the launch of Cataclysm. There is however a workaround/exploit that allows you to obtain it regardless of this.
- Don't Want No Zombies on My Lawn
Survive 15 waves on Endless Mode the Peacebloom vs. Ghouls quest in Hillsbrad Foothills.
I've searched for this but have not found it posted here before. A pre-requirement is that you have not completed the quest series before. If you have, then I am sorry. I do not think you can obtain it.
Step 1: Travel to Hillsbrad Foothills and complete the Basic Botany, Flower Power, Gouls Hate My Grains quests of the series. It is very important that you do not complete "Someone Setup the Pumpkin Bomb" quest. It is probably also possible to complete this on the "Lawn of the Dead" quest but I did it on the earlier one.
Step 2: Start the quest "Someone Setup the Pumpkin Bomb". As soon as you have started it disconnect. By pulling the cable. I disconnected by pulling the cable when confirming it but by accident(net went down for a second) when discovering it.
Step 3: Re-connect as soon as you can. The quest should now be failed but you should still be playing. The event will now be in endless mode. Continue playing it by any decent guide, f.ex: Peacebloom VS. Ghouls - The Strategy Guide to complete the achievement. After surviving 15 waves you will obtain the achievement. Congrats.
*edit* It seems to bug for some people when reaching 6/7 or 7/7 waves where the waves just stops spawning. If you are stuck at 6/7 or 7/7 for a couple of minutes without additional spawns then I suggest you try again.
*edit* Added video - does not show getting the ach as I already had it on the char I recorded on;
*edit* You can also get the "Bloom and Doom" FoS;