This achievement can be missed.
*************!!!!!!!!!!!!SPOILERS!!!!!!!!!!!!!!!************
It must be completed before leaving for Cliboe's dimension.
1.When you first arrive at the Camp located before oasis, talk to the man in the South East. He will ask you to deliver Lena's letter to Verahansha.( I did this step right away, not sure if it will disappear later. So do it ASAP)
2.In Verahansha (timpo), enter the bar, speak with the Schillers and become very confused as to what to do next....
3. After obtaining the Meteomobile, use it to travel to the camp and speak with the most south eastern man to receive Arburus' ring. What is going on ?!?!?
4. I almost missed the optional dungeon in the Ice Castle. The staircase south of the save, guarded by a penguin( did not leave for me until I had acquired Arburus' ring.
Make your way through the Ice Caves until you reach some items frozen in the ground. Arekoll should enter upon inspection of items and make sense of Arburus' and Lena's story.
5. Use the Meteomobile to travel to URG. When you arrive, Arburus will enter the building. Follow and you will find him in one of the rooms. (sorry for the vagueness, just can't recall exactly where. I also used Arburus' ring in all rooms until he was following me.) Lead him to the chapel? room, where he will wait. I did not know what to do here, so I ended up going around using Lena's letter and saw that she had joined my caravan at some point. Return to the chapel and achievement unlocked. Also the best gear for one of your characters!