Two Worlds II

This is not so much of a solution as just a matter of reference.

As it is awarded for completing the Main Story Line of the expansion, the achievement is unmissable (assuming, of course, you make it through to the end --- some parts are a nightmare to get through).

The achievement unlocks upon completion of the Cartographer's Mind quest. There are 3 main sub-quests for this --- Subconsciousness: Conflict of Generations, Subconsciousness: Worldviews in Conflict, and Subconsciousness: Family Conflict. The order in which you play them is up to you (as you pass though a choice of three doors --- each starting a different sub-quest).

After completing each of the three sub-quests, you must defeat Nanook's subconscious "defense mechanism". To defeat this giant, you must make his health get very low (less than 1/4). When it does get low, 4 altars with glowing crystals appear. Destroy one altar at a time and repeat the beating of the giant and destroying altars until he is ultimately defeated.

Afterwards, you'll progress to Nanook and have a nice conversation with him. If you chose "happiness" during the dialogue, you'll be taken back for the treasure chest (and The Pursuit of Happiness achievement). Otherwise you'll get the achievement and the ending credits.