I firmly believe that if Trump raises Bill's infidelities at a future debate, it gives Hillary the chance to effectively end the presidential race. I do not offer this view up lightly, or believe that how I hope she responds would be easy, because fundamentally what I think she needs to do to DESTROY Trump on this issue is let down her guard, take off the armor that she has been very much justified in putting on over the years, and give America a glimpse of the personal challenges that this issue has caused for her.
Since I'm wary of trying to spell out, word-by-word, I'll just outline the key points. Before I do, let me preface by saying that, it goes without say, that Hillary's response should of course reflect what she actually feels. This isn't about putting on a performance. Quite the opposite. It's about allowing for the possibility of her connecting with the American public (and women in particular) in a way that both shames Trump for being such a hypocritical ass for raising the issue, as well drive a stake into the heart of the BS narrative that she is a cold, calculating, untrustworthy, ambition-driven robot that can't connect with the voters.
First, as Michelle said, Trump goes low, we go "high." She should not mention any specifics about Trump's own personal travails, other than perhaps to note that there is plenty of information out there if one was inclined to to look for it. Trump wants (expects) to drag Hillary down into the mud. As tempting as this would be, very little would be gained, and an opportunity would be lost. In any event, Trump raising the issue would give a green light for the media to go after Trump's personal life (including hopefully his own ex-wife's allegations of rape).
Second, there is NO need for Hillary to address any specifics or allegations about Bill's behavior. She shouldn't mention a single person's name, whether it's Monica, Flowers, or whomever. Hillary is running for President, not Bill. All she needs to focus on is the impact it had on her emotionally, and most importantly what she did to overcome it, on her own, as well as with Bill (and Chelsea). She is not responsible for explaining Bill's actions. She also shouldn't be shy about stating that she is a private person, who (like just about any normal person) finds it difficult to talk about personal issues like this.
Similarly, other than perhaps the first line of her comment, there is no need for Hillary to even mention Trump. This is not about responding to Trump, but opening up, and making a connection with the American people. If she is able to lower her guard to show the pain of this issue, viewers will automatically make the connection that it's Trump's fault for deliberately tearing at this old wound, and will punish him accordingly. If she needs some emotional support during her remarks, she could focus on talking to a few audience members (particularly older women), as if she was talking one-one-one or in a small group, which we know she excels at. The town hall format, where she can move around and get close to the audience is ideal for this kind of approach.
Hillary could acknowledge the justifiable temptation at the time to cut her losses and end her marriage, but that she believed it was worth fighting for, for her love and respect of Bill, for Chelsea, and belief that anything of great value requires commitment and determination to make it through the bad times. (This will be a stomach punch for the thrice-married Trump without even saying his name). Here I think a reference to the wonderfully personal speech by Bill at the Convention would work well. For all the complexities of their relationship, there seems to be genuine caring and respect between them. (But I digress.)
Hillary has somewhat reluctantly talked publicly in the past about the importance of her faith, and I don't think it would be a stretch to imagine that it played a role in overcoming these difficulties. If that's the case, then she should mention it. I myself am an atheist, but I recognize the important role faith and prayer have for so many Americans in overcoming life's tribulations. Hillary referencing this contributes to the connection she can build with the public (and also offers a unsaid counterpoint to Trump, who NO ONE believes has a devout bone in his body, including religious conservatives). Similarly, the grace in forgiveness is something that will also resonate with many.
I'll conclude by saying that I COMPLETELY understand if Hillary doesn't want to go "there." No one has the right to press her to do something about an issue that is fundamentally no one else's business, and I won’t begrudge her choosing to brush off the issue with a concise, sharp rebuttal.
But. If she is willing to go "there," and speak from the heart, it could be the most powerful moment of the election. Time would stand still during the debate. The audience would be holding their breath. The moderator would say nothing, with producers screaming through his ear piece to not say a frickin' word, knowing that this would be "THE moment" everyone will talk about. With a split screen, Trump would get slammed if he so much as breathes loudly, and would just have to sit there and take it, for as long as Hillary wants to talk, shrinking as he sees his chances wither away and realizes deep inside his reptilian brain that he walked right into a trap of his own making. It would be like when Trump destroyed Cruz in the debates when he turned the latter's comments about "New York values" into a testament to New York's resilience after 9-11, by the end of which Cruz was clapping. Only take that and multiply by a factor of a hundred.
Game. Set. Match. It would be YUUUGGE.