Story related can't be missed
*** Spoiler - click to reveal ***In Act 2, after having Sylvando rejoining your party, your party gonna suggest you to investigate the Warrior's Rest Inn. Take your ship to Zwaardsrust - Sea and after a cut scene when you will be close by you gonna on land. Make your way to the Inn and pass the night there to have the dream that everyone get there.
Then you can make your way to the Dundrasil Ruin to find an underground passage (same area where Rab and Jade joined your party the first time). Once underground (in the dundrasil castle - Dungeons) make your way to the knight. After you defeat it (shouldn't give you any trouble : I was level 39 with the hero and 37 with the rest of the party), get close to him and listen to him. You gonna get pulled back in the past, enjoy the show and the story then when you get out you are gonna fight the gloomnivore. Just be aware of his attack and use Rab and Sylvando to heal as much as you can and you should defeat it without much trouble again. Enjoy the cutscene following and the achievement should pop.