Can Gust be used on an attacking Immortal Phoenix to remove it before it gains its +2 might from Assault?

Ruling: You cannot Gust an Immortal Phoenix that is attacking, because it gains its +2 might from Assault passively and immediately, making it 5 might before any window to respond. Gust can only target units with 3 or less might.

Nuances:

• If the Phoenix moves to an empty battlefield (not attacking), it remains 3 might and can be Gusted
• If the Phoenix moves alongside a unit with a move trigger (like Traveling Merchant), you can respond to that move trigger and Gust the Phoenix before the showdown begins, since it hasn't been designated as attacker yet and doesn't have the Assault buff
• Move triggers resolve while units are on the battlefield but before showdown starts, creating a window where the Phoenix is still 3 might
• If you Gust away all units during a move trigger response, no showdown occurs