If I enter a battlefield, and pass focus to my opponent, he plays Discipline, can I play Punch first after? Before the Showdown resolves?

Yes, you can play Punch First after your opponent's Discipline resolves, provided the showdown has not yet closed.

The sequence of events is as follows:

1. Initial State: You enter a battlefield, starting a showdown. You have Focus and Priority.
2. Your Pass: You pass Focus and Priority to your opponent.
3. Opponent's Action: Your opponent plays Discipline as a [Reaction]. This creates a chain.
4. Resolution: Both players pass priority, and Discipline resolves.
5. Focus Shift: According to Rule 343, when the last item on the chain resolves during a showdown, Focus passes and the next player gains both Focus and Priority. Since the chain is now empty, Focus and Priority return to you.
6. Your Action: Because the state is now "Open" (the chain is empty), you may play Punch First as an [Action] spell.

Important Nuances:

• Action Timing: [Action] spells like Punch First can only be played when the state is "Open" (i.e., when there is no chain). You cannot play it while Discipline is still on the chain.
• Showdown Closure: The showdown only ends when both players pass Focus consecutively while the state is Open (Rule 344.3.a). Since you are playing an [Action] spell, you are starting a new chain, which keeps the showdown open.
• Reaction Timing: If you had a [Reaction] spell, you could have played it in response to Discipline while it was still on the chain. However, because Punch First is an [Action] spell, you must wait for the chain to empty and for Focus to return to you.

This interpretation is based on Rule 343 (Focus passing after chain resolution) and Rule 344 (Action spells starting a chain in an Open state).

Note: I could not fully verify this ruling against the retrieved rules, as the card type for 'Punch First' was not provided in the context.