Well, Dark Sakura can also take on a servant, and she possesses several of them. As for Atlas, perhaps they would be more prepared.
Honestly, though, I don't really know if Dark Sakura could take down the Association if they went after her. You're probably right that, if they went after her and knew what they were up against, they could kill or (worse) capture her. However, if she went after them without them knowing she was coming or without them realising what she was capable of, she could easily kill the lot of them before they even realised what was happening.