Can you play Janna in a showdown where you are the attacker to that battlefield?

Ruling: No, you cannot play Janna to a battlefield you don't control when you are the attacker unless you have another card or effect that specifically allows you to play cards to battlefields you don't control.

Nuances:

• Janna would need specific text written on the card itself (like Rengar has) to be played while you are an attacker to an enemy-controlled battlefield
• Other cards or effects could potentially enable this interaction if they specifically allow playing cards to battlefields you don't control