To get The Medium Place achievement you will need to get The Lute of Life and escape the Maelstrom. For this you need to have lost the game after going through The Nymphant quest. When starting over, recruit Isabeu and Karst, ignore Leo for now and then do The Rescue, The Escape, The Advocate, The Herbalist, The Blockade, The Liberator and finally The Sacred Temple.
In my run I gave the true Lyre to Lady Katell, cured Volepain of Beastality and had Isabeu slain by Inkblood. You can't get this achievement if you've already enlisted Timothy's help and discovered his true identity.