Shen, Kinkou

Showdowns transition to combat as soon as the state opens, meaning you might not have a chance to respond at action speed while units are outside of combat with cards like Hextech Ray to avoid the shield of Shen Kinkou.

323.12. 8a. If a Showdown and Combat are staged at the same Battlefield and the turn player chooses to initiate the Showdown there, the Showdown will open as a Combat Showdown. 323.13. 9. If the current state is a Neutral Open State and Combat is Staged at one or more Battlefields, the Turn Player chooses one of those Battlefields. Combat begins there. 323.14. 9a. If the current state is Showdown Open State and Combat is Staged at a Battlefield where there is a Non-Combat Showdown ongoing, that Showdown becomes a Combat Showdown.