Does Blitzcrank's ability target, and can you choose a unit (like Shen) that was played in response as the target?

Ruling: Blitzcrank's ability does target. You target as the ability enters the chain, not when it triggers. You cannot retarget to a unit played in response (like Shen) because the target is chosen before opponents can respond.

Sequence:

• Blitzcrank's ability triggers and becomes a pending item
• When pending items are added to the chain, you choose the target as the ability enters the chain
• After all pending items enter the chain, the controller of the last chain item gains priority and can respond
• Once the ability is on the chain, you cannot change targets unless an effect instructs you to do so

Nuances:

• The target is locked in before opponents have priority to respond, so units played in response cannot be chosen as targets