Battle maps are hidden in battle zones and can be discovered by using Examine under the Action sub-menu when a character is one non-diagonal space away from the correct tile. The maps will be listed in order of discovery. In order to unlock this achievement, you must win all the hidden battle maps, not just discover them.
Once you discover and win the first hidden battle map, you will unlock "Explorer".
If you miss it or choose to skip it during a story battle, immediately enter the Skirmish for the zone afterwards and acquire it that way. Once you get the map and unlock the new zone, you may enter that zone and fight a harder than normal group of enemies for an exceptional piece of equipment. I advise you do so as this gear is good for the entirety of the game.
Once you complete "The Battle of Thane," the achievement will unlock as long as you have visited all eight optional battle zones.
Note that they all MUST be visited BEFORE you visit the Gillbaris Research Facility or you will miss the achievement.
Battle Map #1: Dry Riverbed (unlocks Swordsmen Spring)
- In the tree up on the hill by examining the open part of the trunk.
- Treasure: Graceblade (Sword, 16-24 damage, chance on hit for +35HP heal)
Battle Map #2: Ruins of Tolby (unlocks Ragnar's Gorge)
- In a small crate on higher ground across the road from a treasure chest, beside the group of four bandits.
- Treasure: Blood Drinker (Dagger, 20-26 damage, chance on hit for all damage to be returned as health)
Battle Map #3: Dread Caves (unlocks Trivishim's Corridor)
- To your party's right hand side when you start the battle will be a switch beside a mine cart (the one that does NOT end the battle).
- Treasure: Rasmus (Hammer, 16-26 damage, chance on hit for extra damage)
Battle Map #4: Ancient Ruins (unlocks Keliask's Tomb)
- In a stone tablet found between the two groups of enemies.
- Treasure: Titan Helm (Headgear for Luce/Gren, HP+25 DEF+7)
Battle Map #5: Royal Courtyard (unlocks Halls of Atonement)
- In the King's throne at the back of the room.
- Treasure: Runic Mask (Headgear for Tobias/Calvin/Altyria, MP+25 DEF+6)
Battle Map #6: Biruni University (unlocks Foreign Quarter)
- In a barrel to your party's immediate left upon starting the battle.
- Treasure: Celestial Guard (Chestpiece for Luce/Gren, HP+40 DEF+16)
Battle Map #7: Church of Restoration (unlocks Avery Fields)
- In the well on top of the hill, beside the church
- Treasure: Blight (Bow, 25-40 damage, chance on hit to poison; over 25,000 gold)
Battle Map #8: Tiamon Vale (unlocks Gillbaris Gardens)
- In skeletal remains on the opposite side of the tree your party starts beside.
- Treasure: Sincleaver (Axe, 24-42 damage, chance on hit to bleed)