Mythical Gigas appears as a sub-boss found on the 160th floor of Tartarus from Persona 3. It impedes the player's progress and must be beaten to proceed. This boss is strong against all Physical attacks, drains Fire, and has High Counter, which means that physical fighters are very ineffective. The special weapon Gae Bolg, which is a special weapon gained by fusing Cu Chulainn with a Nihil Weapon and changes your basic attack's affinity to Wind is useful in this fight. Otherwise, multiple healers plus a strong mage combo is highly recommended. Using Ice or Electricity to potentially inflict the Freeze or Shock status will also leave it open to critical hits from Slash skills, potentially creating an opening for All-Out Attacks.
Mythical Gigas appears in Heaven's 2nd Paradise within the Midnight Channel. It attacks physically and despite not being weaknesses, it will take double damage from Fire, Electricity, and Wind skills. Upon being defeated, it may drop a Claw of Myth. Selling 4 Claws of Myth will unlock the Assault Spike weapon for Teddie in the shop.
In Golden, it appears in Paradise 1 and 2. It may drop Meteorite for trade at Shiroku Pub.