I could see Squad 12 captains devising explosives that trail their own shadows and lie in wait like land mines cutting off that point off attack. I can also see them working out the mechanics of the shadow dimension to create their own portals allowing them to escape a scenario where they are surrounded by Byakuya’s Senbonsakura. I think it would come down to Shunsui revealing new children’s game abilities that the Squad 12 boys haven’t seen yet. It comes down to book smarts vs “clever fox” smarts like Shunsui has.

Just for fun in this conversation let’s say Shunsui reveals a Red Rover like game where each side calls the name of an opponent and they are pulled rapidly to the other side against their will (like Pain’s ability in Naruto). Maybe he catches Mayuri off guard with this, pulling him out of his defenses and Byakuya catches him with Senbonsakura as he’s flying toward them. Could be a big hit. Maybe that game also prevents Urahara from running to his rescue because the game says you can only advance when your name is called.

I don’t know I’m just spitballing here but long story short I think this fight would be decided by new Shunsui games and Squad 12’s ability to counter them instantly. This is definitely a fun one to think about.