Roughly Which Bahamut Raid Requires A Premade Group?

[Sibyll]

I expect to see lots of people defeat T10 and T11 in DF in the next few days.

T10 because it's really not too hard to learn your individual duties. Minor amount of teamwork and coordination.
T11 will be harder, but I expect a high success rate mostly, because people will run it a lot more often, as it drops carboncoat and encryped tomestone.

You can get carboncoats from the WoD weekly. I wouldn't be horribly surprised if Tomestones and Carboncoats (and probably carbontwines as well) become drops in WoD considering they did that for ST. If that ends up being the case then people will just spam WoD instead of T11. It's just easier, albeit more people to roll against.