Firstly, I've found Abrupt Decay to be very good against Xerox, assuming you run a deck that supports those colors.
Secondly, this is going to be a tough thread precisely for the reasons that Chubby pointed out. Xerox is often, if not axiomatically, situated in this midpoint within the meta. It is the quintessential mid-range deck, and it's very tunable. Against almost any deck that's trying to go fast, Xerox is going to be the control. Against any control deck, Xerox is the aggro. It's the multirole aircraft of the format, this mercurial blend (stew) of blue cards and tunable flex slots.
That said, I think there ARE more than a few things to be discussed in this thread that can benefit players in games against Xerox. My current theory is that there are basically three methods that beat this deck. Or said a different way, when you beat this deck, you almost always do it by achieving one of three things: Kill them fast, Kill their tempo advantage, or Kill them with card advantage.
(A note on what "good" means. I actually love Xerox even though I rarely play it, because I think that it really rewards good play, both by its pilot and when played against it. One of my favorite matchups to play either side of is a Control Oath build vs. Xerox. There are so many lines and choices that win or lose the games tend to be a lot of fun. Therefore, it's a little more problematic when people speak in terms of X deck "beating" Y deck. I guess we can define one deck "beating" another as having an advantage over time given perfect play by both players... but ok.)
Kill them fast: Dredge tends to give Xerox problems because its difficult to control and kills Xerox much faster (on average) than Xerox can kill Dredge. PO similarly can have a good matchup depending on tuning, for the same reasons. Oath might be a distant 3rd here, but this is very build dependent as both Xerox and Oath have a ton or room to tune. But the point is that if you are playing one of these decks, or a deck like them against Xerox, then the correct lines might just be everything that puts max pressure on the Xerox deck to have answers in the first 2-3 turns. Xerox is not among the fastest decks in the format. Recognizing that and even boarding out Forces in favor of threats, or taking down your shields to try to kill fast can be correct.
Kill their tempo: This is a way for things like Shops and perhaps Standstill or Humans to fight against Xerox. With the exception of busted Shops draws, I don't find this to be particularly effective, but again, the point here is to evaluate methods for finding the correct lines and cards, not to say what is "good." Recognizing this line of attack against Xerox explains why resolving a Chalice at 1 might be better than resolving 2 or even 3 creatures if you are a Shops player. When the Xerox deck can't see a bunch of cards each turn, it gets much much worse.
Kill them with card advantage: This is how control decks typically fight Xerox. Standstill, Big-Blue, Big Mana Thieves, and some Oath. One thing to recognize about Xerox is that while they have a ton of cards that have "Draw" written on them, very few of these actually net cards right away, or at all. Fighting an attrition war with Xerox can be the correct line. Imagine whittling down the cards in hand for example, then resolving a simple Standstill. Recognizing that sort of thing will lead us to correct plays such as intentionally running an Oath into a FoW to gain a simple 2 for 1. Or not spending mana to attack with Mishra's Factory, where the damage/tempo is meaningless, in order to hold up mana to pay for a potential Flusterstorm.
Lastly, notice how much overlap there is between these roles. Often Shops is simultaneously trying to kill the Xerox deck's tempo advantage, while killing Xerox quickly. Often Oath or Theives is trying to get ahead on cards while killing fast as well. Recognizing which axis can best cause you to win is vital in these matchups, and it explains why skilled players on either side of the matchup can tip the "luck" so steeply in their favor. And whether you are favored or not given a matchup or build, all you can do is try to select those choices that maximize your chances of "getting lucky" and beating Xerox.