There are six gigas, one under each color of the moon. The gigas are very powerful, but that goes without saying... Because they are so huge, Vyse and company cannot find them on the deck of their ship or on land. Instead, the player will always fight them in an airship-type battle, firing cannons and all. Some gigas Vyse will have to battle, one he will never get to battle, two he cannot kill, and some he can actually defeat.
The red gigas, is the first gigas that Vyse will face. Recumen cannot be killed, not even a harpoon cannon makes a dent in it. The only thing Vyse and company can do is survive until another option makes itself known. It's not too difficult to survive against this gigas. After a while, the Little Jack will take massive damage, and the group will decide to go after Belleza, who summoned the gigas, and fight her flagship, the Lynx. By defeating Belleza and taking the red moon crystal back, Recumen will bury itself in the sand.
The green gigas, is the second gigas that Vyse will face. Grendel can be defeated in one of two ways. After battling the gigas with normal cannons to do enough damage and surviving with the use of repair or deluxe kits, the gigas will start holding its head as though it were confused (and it probably is because the one controlling it is knocked out) and the player will be prompted to fire the harpoon cannon, knocking it into a gorge. However, if the party is at a high level, then they can skip firing the harpoon cannon when prompted and reduce its HP to zero by using fighting normally. Of course, the ending animation will show Vyse using the harpoon cannon either way...
The blue gigas, is the third gigas that Vyse will face. This battle should be much easier because Vyse will fight this gigas using the Delphinus, which is much sturdier and more powerful than the Little Jack. Considering that Blueheim is the gigas of the blue moon, which has power over water and wind, it is no surprise that this gigas is so fast. It should not be anything special if the party is high-leveled.
Rhaknam (or Plergoth)
The purple gigas, is the fourth gigas that Vyse will encounter. Vyse and company never fight this gigas because Ramirez seriously wounds it with his flagship, the Monoceros. They meet this gigas in Glacia, Rhaknam's home, where they see Drachma again after his assumed "death". Drachma explains that if the Little Jack wasn't attached to Rhaknam as the arcwhale fled, then he wouldn't be alive, so Drachma couldn't get revenge. As it turns out, Rhaknam used to be a normal arcwhale, but the people of Glacia embedded the purple moon crystal into it and mutated it into the massive gigas it was today. Rhaknam is dying, and Drachma decides to stay with it in its final moments when he realizes that Rhaknam was also a victim of circumstance. When Rhaknam dies, the purple moon crystal falls out of its corpse.
The yellow gigas, is the fifth gigas that Vyse will face. It sleeps in the Maw of Tartas where it is was sealed. No one could break the seal from above, but it can be accessed by sailing underneath Valua. Of course, once Vyse and company reach Yeligar's place of slumber, they wake the gigas up, and it easily tears the seal on the Maw of Tartas, releasing itself. Vyse and company then engage it in battle.
The silver gigas, is the sixth and last gigas that Vyse will face, and it is by far, the most difficult gigas the player will fight. Fina will explain to the group that Zelos is based on biological constructs like Cupil, able to change into many shapes. It is still forming, so she advises them to start attacking it now before it develops completely and becomes invincible. Zelos is noted to be so strong that it takes five moon crystals to break its seal, and summoning Zelos allows the summoner to call down the Rains of Destruction, a weapon that can wipe out even the most formidable of countries, like Valua. Useless trivia: This is the last gigas the player faces and the last airship battle they fight in. Zelos will also drop a Captain's Stripe when defeated, but it is utterly useless because there is no save point at the end of the game. Zelos is also the only gigas that consumes and merges with a boss, that boss being the Silvite, Ramirez.