It is not possible to do this achievement because you automaticly get phased into a 120 lvl zone with Magni giving you a “Heel ye Woons” quest. The tent and the quest for the midsummer event is gone.
Phasing in most zones are caused by quests or the progression your character has made. These can normally be reset by finishing the specific quest lines in those zones, or by speaking with Zidormi.
As you didn’t mention a particular zone, but specifically called out the Cataclysm version of the achievement, only Hyjal, Uldum and the Twilight Highlands should be affected by this. Hyjal and Twilight Highlands can be due to quests related to your Heart of Azeroth, and can be fixed by finishing those quest lines. For Uldum, Zidormi will be available to your character and speaking to her will allow you to see the pre-invasion version of the zone.
That should hopefully make it easier to complete the achievement.
Thanks, it was Hyjal, sorry for not mentioning it.
I did the quest and tried to go in and out of the zone but nothing happened. I have now finished the whole questline, I will test this.