Does Counter Strike work against damage from the last step of a Combat Showdown?

Yes, Counter Strike works against damage dealt during the Combat Damage Step of a combat showdown.

According to Rule 460.2, the Combat Damage Step is when units deal damage to each other. Because Counter Strike creates a delayed replacement effect that waits for the "next time" a unit would be dealt damage, it will successfully intercede and prevent that damage when the combat damage is dealt.

Key Considerations:

• Simultaneous Damage: Combat damage is dealt simultaneously to all units involved in the combat (Rule 460.2.c.1.a). Therefore, if you target a unit with Counter Strike, it will prevent all combat damage that would be dealt to that specific unit during that damage event, regardless of how many opposing units are attacking it (RiftJudge FAQ #8527).
• Timing: You can play Counter Strike as a [Reaction] during the combat showdown. If you play it while the chain is open or closed (e.g., in response to an ability or simply during the showdown), it will resolve, create the replacement effect, and wait for the damage step.
• Replacement Effect: As a replacement effect, it alters the execution of the damage event (Rule 369). Once the damage is prevented, the effect is consumed (RiftJudge FAQ #8921).

Summary: If you play Counter Strike targeting your unit before the Combat Damage Step, it will prevent all combat damage that unit would take during that step.