Mythical Gigas appears as a sub-boss guarding the way forward on the 160th floor of Tartarus from Persona 3. It is neutral to Distress, has a minor weakness to Rage, and is immune to all other ailments.
This boss is strong against all Physical attacks, drains Fire, and has High Counter, which means that physical fighters are very ineffective. The weapon Gae Bolg, which is a special weapon obtained by fusing Cu Chulainn with a Nihil Weapon, has an affinity to Wind and is useful in this fight. Otherwise, multiple healers plus a strong mage combo is highly recommended. Using Distress, along with 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.
Persona 4 / Golden[]
Mythical Gigas appears in Paradise 2 of Heaven within the Midnight Channel. It attacks physically and despite not having any weaknesses, it will take double damage from Fire, Electricity, and Wind skills. When defeated, it may drop Claw of Myth. Selling four Claws of Myth at Daidara Metalworks will unlock the Assault Spike for Teddie in the shop.
In Golden, it appears in Paradises 1 and 2. It may also drop Meteorite for trade at Shiroku Pub.