My opponent's Irelia champion is at 3 might. They declare En Garde targeting Irelia and tap Irelia legend on target. Can I then react with Gust before Irelia readies and gets +1 for target?

Yes, you can react with Gust to attempt to return Irelia, Fervent to their hand before she readies.

However, there is a crucial timing mechanic you must understand regarding the chain and how your opponent's play sequence resolves.

The Sequence

1. Playing En Garde: When your opponent plays En Garde and chooses Irelia as the target, two triggers are added to the chain simultaneously:
   • Irelia's Trigger: "When you choose... me, give me +1 Might."
   • Blade Dancer's Trigger: "When you choose a friendly unit, you may exhaust me and pay [A] to ready it."
2. State is Closed: The moment these items hit the chain, the state is Closed. You now have priority to play a [Reaction], such as Gust.
3. Your Response: If you play Gust now, it is added to the top of the chain.

Resolution (LIFO - Last-In, First-Out)

The chain resolves in reverse order of how items were added:

1. Your Gust resolves: Gust checks its targeting requirement (unit must have 3 Might or less).
   • Irelia currently has 3 Might.
   • Gust resolves and successfully returns Irelia to your opponent's hand.
2. Other Chain Items: Once Irelia is in your opponent's hand, she is no longer on the board. The remaining items on the chain (Blade Dancer's ready trigger and Irelia's "when chosen" trigger) will attempt to resolve, but since the unit they reference is no longer on the board, they will "mistarget" or be unable to execute their instructions (Rule 359.3.e.12, 359.3.e.5).

Summary

If Irelia is at 3 Might when you play Gust, your play is legal and successful. Because Gust is a [Reaction], it sits on top of the chain and resolves before the ready effect or the Might-granting triggers can execute. Irelia will return to your opponent's hand, and En Garde will resolve with no target (it will whiff).