If I understood it correctly they needed to shut it down in order to negate the way his HP bar was acting and disable his teleportation ability. You need to keep in mind that they made this plan without Haganemushi, in which case the easiest way to stop him would be to kill him and then collect his sword. It's not like they were aiming to destroy the sword earlier since it didn't seem like they could do it.
As for why they shut down the magic circle at that point, they mentioned it already that the sword gave him buffs for every adventurer within a certain radius so they were kiting him around whittling his HP bar down. If they shut it down straight away and swarmed him they might have suffered more losses because of the buffs and if they were in small groups like now he might have escaped once the armor lost it's properties which is probably why they timed the freeze skill. There's also the fact that they had to summon the Kunie clan to a meeting and have them agree on shutting the field down, there's a possibility that he wasn't in Akihabara and needed to travel back after the last meeting. So basically they planned for different scenarios and were buying time until all the pieces came together. At least that's how I interpreted it all.