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In the book Rule of Two Darth Bane's apprentice, Zannah, meets a follower of the dark side named Hetton who has 8 Sith assassins working for him, modest dark side abilities of his own, and who is rich in Sith artifacts and knowledge acquired by wealth.

Zannah brings the 8 assassins and Hetton to face her master. The novel says that she could not predict who would win the battle.

Bane defeated Hetton and the assassins, then attacked Zannah. Zannah got Bane to stop attacking her (in part) by explaining that she had not turned against him, but that she was delivering Hetton and his guards to be slain by Bane. When asked, she said that if the guards and Hetton had killed Bane then that would've meant he was weak and deserved to die. The book says Bane realized that her actions were in "perfect accordance" with the Rule of Two and his other teachings.

But isn't this the exact situation which both the Rule of Two and the Brotherhood of Darkness were meant to prevent from degenerating the strength of the Sith? Bane had explained that prior to the Brotherhood the Sith had become weak because many weaker Sith would gang up to defeat a stronger master. In this way the Sith became weaker and weaker instead of stronger and stronger.

At this point in the series Zannah was weaker than Bane, who still had his Orbalisk armor. Hetton was weaker than Zannah and the average assassin guard was presumably the weakest of the group. So, if Hetton and the assassins had killed Bane wouldn't that have made the Sith weaker and wasn't Zannah therefore breaking the Rule of Two?

While you are correct that Zannah wasn't completely sure Bane would win, the book indicates that she expected him to win fairly easily:

Her Master easily dispatched Hetton and his minions, as she had expected...though there had been a brief instant near the start of the battle when Bane had appeared vulnerable.

_{p. 205}

The above quote refers to her private thoughts so there's no reason to suspect she didn't actually believe Bane would win easily. Zannah told Bane much the same thing when Bane ended up approving of her actions:

I knew you had the strength to defeat them, Master,” Zannah said. “That was why I didn’t come to your aid during the battle.”

“And what if you were wrong?” Bane asked in a quiet, menacing voice. “What if they had somehow killed me?”

“Then you would have been weak, unworthy of being the Dark Lord of the Sith,” Zannah answered boldly. “And you would have deserved to die.”

“Precisely,” Bane said with his familiar grim smile, and Zannah knew her Master approved.

_{p. 207}

Zannah's ambush does seem to violate the Rule of Two because the apprentice is supposed to defeat the master in personal combat and without any help from anyone else. However, it is important to remember another of Bane's teachings which is referenced in Bane's conversation with Zannah:

By its very nature, the dark side invites rivalry and strife. This is the greatest strength of the Sith: it culls the weak from our order.

_{Darth Bane: Path of Destruction, p. 237}

I think this principle -- that the weak must be culled and therefore deserve to die -- is necessary to reconcile Zannah's ambush with the Rule of Two. The Rule of Two is designed to prevent a group of Sith apprentices (as in, trained Sith) from prematurely killing the master. However, Hetton had not received any formal training from an actual Sith and Hetton's eight assassins were

former students of the Sith Academy on Umbara

_{p. 197}

If an untrained practitioner of the dark side and a group of former students were strong enough to defeat Bane then Bane would have been pathetically weak and would have needed to die.

A minor nitpick: Bane didn't actually say that Zannah acted in "perfect accordance" with his teachings -- that's what she concluded:

Zannah had saved herself by explaining the motives behind her actions. She had acted in perfect accordance with her Master’s teachings—a fact Bane should have realized on his own.

_{p. 216}