@algeco Unlike Class Trials, you can use any class of heroes in Challenge Events.
However, there are three tiers, each with limits on hero and troop rarity:
- Rare: 3* or lower heroes, 2* or lower troops
- Epic: 4* or lower heroes, 3* or lower troops
- Legendary: 5* or lower heroes, 4* or lower troops
Additionally, there are limits on allowed items.
For more information, see Memento 4 Events