If I control both battlefields and my opponent attacks one, can I play 'Here to Help' to play Blitzcrank at my other battlefield, and would his ability resolve during the contested showdown to create a second showdown?

Ruling: Blitzcrank's trigger goes on the chain during the ongoing showdown. Once it resolves and moves the unit, it stages a second showdown that waits until the first showdown finishes before proceeding.

Sequence:

• Opponent attacks one battlefield, starting a showdown
• Play 'Here to Help' and Blitzcrank at the other battlefield
• Blitzcrank's trigger is placed on the chain as a reaction
• Blitzcrank's trigger resolves, moving the targeted unit
• The moved unit stages a second showdown
• The second showdown waits for the first showdown to complete before proceeding

Nuances:

• If you move an attacking unit like Yasuo with Blitzcrank, the unit gets a second attack trigger at the new battlefield
• The first attack trigger can resolve without the unit present, but won't deal damage since the unit is no longer 'here'
• Referents are evaluated on resolution, so effects check where units are when they resolve, not when they trigger
• If you hook Anivia in response to her trigger, she would deal damage to units at the battlefield Blitzcrank was played to once the move resolves and her trigger resolves