The Susano'o arm leaves too much space for Sasuke to be attacked. He can throw a kunai inbetween the gap, if Sasuke throws a kunai to block it, Minato can spread them, teleporting to each one, one by one. Sasuke will be confused, and he won't know wich one he'll appear from. Minato can then proceed to expose that gap. If he chooses to block it with the arm, he's even more vulnerable than before.
Or Minato can just tag the ribcage... Minato will appear right infront of Sasuke, who won't know what the heck just happened, and bam, he's gone.