After completing the 10 main story lines and the four chapters of Shade Impulse.
The Duel Colosseum is unlocked after completing all of the main storyline and Shade Impulse; however, it is not tied to the story in any way. In this mode you will be presented with cards (three different types in all), and you can select them to perform different actions. The Duel Colosseum a great way to get equipment or to upgrade previous items, as well as to level up Chaos characters (since even level 1 characters can enter it effectively).
First, pick the character of your choice. You may choose either a hero or a villain. Next, pick the course you want based on level range. The level range corresponds to the possible levels that you may encounter while playing in that course. For example, if the Lunar Whale Course is chosen, one can expect to fight characters ranging in level from 91 to 120. After the Colosseum is chosen, a screen with cards will be shown with a selection area and job card area. At this menu, you may change your equipment or leave by choosing EXIT.
Points are given to the player for the amount of rounds played (each card chosen progresses to another round) and the amount of medals the player has. Upon exiting the Duel Colosseum, the player receives all items acquired up to that point and receives PP proportionate to the amount of medals earned.
Selection Area - The area where you can pick the cards. You are granted 3 spaces in the selection area by default. If one is to lose in a fight with an Enemy Card, you lose one space until you can no longer pick an option other than EXIT. At that point, you must leave the course and might try again the same or different course.
Card Luck - Card Luck refers to the quality of cards that you receive in the selection area. With one star worth of Card Luck, players may see Enemy Cards worth 1 medal, while with full Card Luck, it is common to see 7 or 8 medal Enemy Cards. Card Luck is achieved randomly by defeating Enemy Cards, or by certain job cards.
Enemy Card - Select an Enemy Card to fight that enemy. When selecting an Enemy Card, it will display information such as the name, level, and strength of the enemy, the arena in which the fight will occur, the ruleset of the battle, and how many medals you can win or lose in that battle. By defeating or losing to Enemy Cards, you can gain or lose medals, which can be used to buy Treasure Cards. Losing to an Enemy Card will cost a Selection Area space. One can tell what enemy is on the card before it appears in the Selection Area by the silhouette of the crystal at the back of the whitish blue card, but it could be either the hero or the villain.
Boss Card - A golden card seen before appearing in the Selection Area is a forewarning of a Boss Card approaching. Boss Cards are not discarded from the selection area after the player makes a choice of any card, with the exception of the Samurai Card, which discards it. Like Enemy Cards, Boss Cards are special fights with chosen opponents that yield medals. The strength of Boss Cards are Very High. In general, Boss Cards yield more medals than a regular Enemy Card of the same level.
Chaos Card - The character fights Chaos with the strength Chaotic. This card cannot be discarded by any other means other than choosing the card itself, except by being lucky with a Dragoon Card, Summoner Card, or Samurai Card. Chaos Cards will yield 50 medals by default. Losing to Chaos results in the loss of Card Luck, Medals, and Selection Area Space by default. A Chaos Card has Chaos's stigma at the back of the card and is red.
Special Card - A special enemy card that doubles the player's medals if the battle is won with a "x2" symbol on the face of the card meaning the result will double the player's current medal count as well as the conditions on the top of the enemy's name instead of a + and a random number. A question mark serves as a forewarning. If the player should lose, it halves the current amount of medals, along with reducing Card Luck and Selection Area space. These cards usually have the toughest AI of any cards with the strength Maximum, with the exception of Chaos Cards.
Treasure Cards - Treasure cards offer players unique items and equipment by exchanging medals. Treasure Cards will always have the same value. Also, items that appear in all four Care worth the same amount regardless of the level range, such as Life Stone. Some items offered in Treasure Cards can only be gotten through Duel Colosseum.
Job Cards - Job Cards give you special abilities that affect the other cards. You can have up to three class cards at once, and using specific ones together grants extra bonuses.