Can Fiora's ability be used to ready Leona Champion when she becomes mighty during a showdown after her stun resolves and her Legend ability triggers?

Ruling: Yes, you can pay {O} to ready Leona when she becomes mighty during the showdown sequence.

Sequence:

• Leona moves to a battlefield occupied by an opponent
• Leona's stun goes to the pile and resolves
• Leona's Legend ability triggers and buffs Leona Champion
• Leona Champion becomes mighty
• Fiora's ability can be used at this moment to pay {O} and ready Leona Champion