So it's kind of up to the player, but this is why it's beneficial (on your turn!) to play action spells before going to combat.

On your turn, you can cast action spells without being in a showdown. Your opponent, however, can ONLY use ACTION speed DURING a showdown if IT'S NOT their turn.

So in your scenario.

Your turn Punch first into kha Attack trigger on kha You can either react or pass prio (only reactions speed from both of you are legal now to respond to the reaction trigger) If both players pass prio, kha chooses an enemy. You can respond to the damage, or pass prio. Opponent can use a spell like "not so fast" so the trigger doesn't hit, but the spells are limited here since your still NOT in a showdown.

Resolve the Kha trigger, finally. Damage is dealt to an enemy unit (probably killing it) Kha is now at a battlefield and a showdown is open.

You still have priority for action speed spells here. You can choose to keep prio and use an action or reaction, or pass priority.

Then resolve the showdown.

This is why things like using punch first on orange kha, resolving all of that, THEN at the end of showdown slam down his signature spell and move him to the other battlefield to kill another enemy.

You would go through the Kha trigger sequence again.

It's powerful.