This achievement/feat of strength isn't intended to be obtainable ingame because the limitless setting needed was eliminated prior to the release of Cataclysm. There is nevertheless a workaround/exploit that permits you to acquire it in spite 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 sought out this but haven't thought it was posted before. A pre-requirement is that you simply never have finished the quest sequence before. For those who have, then My apologies. I really don't believe you can acquire it.
Step 1: Travel to Hillsbrad Foothills and finish the Basic Botany, Flower Power, Gouls Hate My Grains quests of the collection. It is crucial you do not finish "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: Begin 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( internet shut 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.
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.