One of the tails does give Zack a good smack, but it's shrugged off because there are more important things, like making sure Angeal doesn't interrupt Martel's spell again (and hoping that said spell works!). He's doing his best, even moving closer to attempt to grapple with the creature if he has to. Maybe a good shake of the shoulders will help; maybe he can shake some sense into the man who seems so disinterested in what the creature around him is doing.

"C'mon, Angeal! Stop this!"