It is easiest to obtain this achievement as the last thing you do on your first playthrough of the game—either after watching the credits, or just with no other missions/quests active.
If you're having trouble "going the distance":
- Drink the Potion of Sacrifice at Mudder's Tavern to lower dungeon danger by 30%. (On a related note, make sure you don't have any other potions active that may increase difficulty.)
- Make an offering at Davies' Priory, and hope for a modifier like Fragile Monsters or Weak Monsters.
- Fully upgrade sword/staff/shield tiles and recruit all monsters.