This achievement is still obtainable in 5.1. To get it you must do the following steps.
1. Make the following macro.<pre>/script SelectGossipAvailableQuest(1)
2. Accpet the daily "Tending the Garden".
3. Click on the leave vehicle button to fail the quest.
4. Open your quest log and click on the quest. Click abandon but don't hit yes on the confirmation box yet.
5. Talk to the NPC and then hit the macro.
If done properly you should now be in endless mode. Last for 15 waves and you will get this achievement! Good luck.