Can you react to Anivia entering a battlefield to increase your creatures' might before they get killed?

Ruling: Anivia does not trigger when she moves into a battlefield. She triggers once the combat showdown starts and she is attacking, with her effect going on the initial chain of a showdown.

Sequence:

• Combat showdown starts with Anivia attacking
• Anivia's effect goes on the initial chain
• You can react on that chain with reactions

Nuances:

• The trigger timing is at showdown start, not when Anivia moves to the battlefield