This Colosseums are called Novice Challenge and Expert Challenge.
The idea of this Colosseum is having to defeat certain opponents:
a) using a deck that has to pass certain filters, like "No pillars", "no creatures", "Only cards from 2 elements", etc.
b) Beating them with certain restrictions, like "Having a creature with more than 30 attack", "Beat it with more than 20 poison counters", "Beat it with more than 10 purify counters on yourself", "One Turn KO", "Have more than 10 creatures in your field", etc.
The Novice Challenge will be against a given Mage and the Expert Challenge will be against a (random) Champion, but 3 tries per day.
There is a certain amount of possible challenges, and only one of them will be spawn per day like every other Colosseum.
Thoughts? I can invent many of them.