Fomoire begins the fight by using Murderous Might, which allows him to use Dominating Fist and allows the player to attack his fist. When the fist is defeated, Fomoire is crippled offensively.
The Fomoire is a powerful foe, but not dangerous if the player cuts off access to his Dominating Fist ability. The player should start by attacking his fist with Relentless Assault (COM/RAV/RAV), bringing up the chain gauge and upping the damage dealt to the fist.
Should the fist become staggered, the player should switch to Cerberus (COM/COM/COM) to deal more damage in a short amount of time. After the fist is defeated the party should begin with Relentless Assault to stabilize the chain gauge, then switch to Tri-disaster (RAV/RAV/RAV) to stagger it.
Once Fomoire is staggered, the player should switch to Cerberus and juggle the Fomoire in the air. The Fomoire will not likely be killed in the first stagger period, so the player may wish to use Meteor Javelin or Ultima Arrow before the enemy has lost its stagger meter fully to deal more damage.
The Fomoire will likely use Murderous Might a second time once its stagger gauge is empty; should this be the case, the party must defeat it again. It will not likely last long past its second or third stagger.
The Fomoire or Fomorians are a race of half-divine beings from Irish lore.