Looking it up, figuring out its a wall of force requires a
dc=25 spellcraft check, I think that includes knowledge of how to bring it down which is method #1 in the sidebar.
dc=30 spellcraft check for unique magical effect, i.e. figuring out that a turning check will work.
Figured if it took me a sec to figure out I'd share the info.
That's what I used when my party hit the area. They didn't see the Apostle itself, only the part of its sphere that stuck out in the eastern hallway. A 30 knowledge(arcana) check gave the wizard the idea to use a turning check (channeling positive energy), but I upped it to 35 for him to also get the second bit about Dispel Magic. I would have also allowed a second check at DC 30 if they only tried to use a turning check to get through.
PS- They made all their turning checks, but didn't make the dispel checks on their first few tries. They had to rest in the next room long enough to get their spells back... suffering all sorts of threatening and doubt-inducing dreams.