The Tunnel Armor attacks using almost entirely magical attacks, casting Thunder, Fire and occasionally Poison and Magitek Laser. Its only non-magic attack besides its normal attack is its special attack Drill, which does high physical damage to a single target. When the fight begins, Celes warns Locke the Tunnel Armor uses a lot of magic, and says she can absorb its spells with Runic.
The player is advised to use Runic with Celes every turn, negating the Tunnel Armor's magical attacks while Locke attacks it. As long as Celes keeps absorbing the boss's magic and Locke uses Potions to heal when needed, the battle will be long but not overly difficult.
A chest in the upper-left portion of the cave contains a Thunder Rod. When used as an item it casts Thundara on all enemies, which against the Tunnel Armor will kill it in a single hit. However, if the player leaves the chest alone it later contains a rare Hero's Ring, so they must consider if forfeiting the ring is worth the easy victory.
Unless the player has saved Locke's scenario for last, it is advised to de-equip him and Celes of any powerful Relics and armor pieces the player has found, allowing them to pass these items on to their party in the next scenario. Due to Celes's Runic, the player does not need high-level armor to be safe, so this will not hinder them greatly.
As the Tunnel Armor emerges from the rear part of the South Figaro cave, where in the World of Ruin a passage leads to the submerged Figaro Castle, the Gestahlian Empire was likely using the machine to try and dig to the castle before it was ordered to prevent Locke and Celes from escaping.