Madeen
You wouldn't think it to look at her, but Eiko has the power to melt the universe. The holy power brought forth by Madeen is on a par with Ark's and, being elemental, surpasses it against the undead. The magic cost is a high price to pay, but just feel that damage. Boost with Boost and the enemy won't know what hit it. Just don't forget to pack some Ethers.
Summoner
Eiko
Gemstone (well, sort of)
Ribbon (120AP to learn) (Yes, 120)
Ribbons may not be very well known for their gemstone properties, but forget about that for a moment and just assume they count. Eiko gains her first ribbon during a pivotal moment in Mount Gulug, after which she should learn to summon Madeen ASAP. Another ribbon may be unearthed from the Forgotten Island chocograph. If needed, and if the party has gil burning a hole in their gilbags, the Treno auction house is a good place to look for a third.
Attack
Terra Homing (54MP)
Causes Holy damage to all enemies.
The ground becomes a swirling mass of energy, before erupting into the sky along with the enemy party. The wolf-like Madeen surveys the scene, his claws trailing energy in their wake. With a roar, he creates a spherical mesh of light, which draws the enemy into it. Just as things are beginning to settle, the ball releases its energy in a massive explosion.
Movie (AVI, DivX, 2.92MB)
Screenshots
