Can Lucian Gunslinger deal damage before Shield apply?

To determine if Lucian, Gunslinger can deal damage before a defending unit's Shield applies, we must look at the timing of combat and how Shield functions.

The Short Answer

No. Lucian, Gunslinger cannot deal damage to a unit "before" its Shield applies because Shield is a Passive Ability that is inherently active whenever the unit is a defender in combat. There is no timing window between the unit becoming a defender and the Shield bonus being applied to its Might.

Explanation

1. Shield is Passive: According to Rule 814.1, Shield is a passive ability. It does not use the chain and it does not trigger. It is "always on" as long as the unit has the Defender designation (Rule 814.1.c).
2. Combat Timing: When a unit is designated as a defender at the start of a combat showdown, it gains the Defender designation immediately. Because Shield is a passive ability, it immediately calculates into the unit's Might as part of the game state (Rule 471, Layers).
3. Lucian's Ability: Lucian’s "When I attack" ability is a triggered ability that goes on the chain when combat begins. Even if your opponent reacts to Lucian's trigger, the defender's Shield bonus is already active because the unit is already a defender.
   • If your opponent plays a card to remove the defender or the Shield, they can do so in response to Lucian's trigger, but they cannot force a window where the unit is a defender but not shielded.

Summary of the Sequence

• Combat Begins: Units are assigned Attacker/Defender designations.
• Shield Activates: The defender's Shield bonus is applied to its Might instantly (passive, no chain).
• Lucian Triggers: Lucian’s "When I attack" trigger is placed on the chain.
• Interaction: If an opponent uses a reaction to remove the Shield or the unit, they do so while Lucian's trigger is on the chain. If the Shield remains, it is already providing its bonus to the defender's Might when Lucian's trigger resolves.

In short, Shield is not a "response" that happens after Lucian acts; it is a static modification to the defender's characteristics that is present the moment combat starts.