As you can imagine, that much precision will usually wreck shop on a single player. It will also wreck shop on most duos. It's not until you get to three coordinated players (or more) that you're ready to go toe-to-toe with a multiboxer. Even then, a skilled MB can usually decimate a large group before you even realize what it is you're facing.
If you find yourself throwing down against a multiboxer, what do you do?
Use the terrain
Most multiboxers are pretty good at what they do. Even if that multiboxer is the Bruce Lee of multiboxers, though, he has to deal with an important reality -- moving five characters around at once.
The second you see a multiboxer, check out your local terrain. Bushes? Towers? Rooms? Start zigging and zagging around all that stuff. Think of this as a hyper-caffeinated version of pole dancing from the Arena.
While most multiboxers are fairly good at movement, they still won't be as fast or agile as a single character trying to thread a needle. But you've got to move like the wind. Climb up that tower, back down those stairs, and move quickly. Your hope is that as the multiboxing characters all chase you, they'll eventually get disrupted among the terrain. Then you can whirl around and disembowel them like the primitive PvP god you are.
Obviously, your opponent will launch a spell at every opportunity, and that spell will likely kill you. Therefore, you're trying to separate those characters. I'd like to say this results in a win all the time, but it doesn't. More frequently, I find the multiboxer just decides I'm more trouble than I'm worth and wanders off.
Lay down some CC
Bust out your stuns, your roots, and all your other CC tools, and be ready to use them. Much like sprinting around terrain like a scared bunny, the real goal is to break the vital coordination that makes multiboxing possible.
The problem here is that unless you're a frost mage, you won't have enough crowd control of your own to break up a determined multiboxer. Snag one, disappear around a corner, wait for the others to follow. Keep sniping and CCing one character at a time and hope they eventually split up.
Does it work? No, not really. Again, the more likely scenario is that the multiboxer will get annoyed and wander off.
Get a friend
Here's the reality of a solo fight versus a multiboxer: You're not fighting one-on-one -- you're fighting one-versus-many. It's just more rankling because you know a single player is behind that 5-man group that turned you into street pudding. A multiboxer does have the advantage of coordination, but the real advantage is that he's hauling around five characters.
Your best response? Get some friends. Get a handful of fellow players to hook up with you, and mow down the offending multiboxer. You'll need to coordinate and still be good at PvP, but fighting a multiboxer with a group is your best way to go.
Does it work? That depends on your skill. Because your opponent has the same target for five damaging attacks at one time, he'll mow down your group pretty quickly. You need to act fast and decisively. That being said, attacking a multiboxer with a group is absolutely your best bet.
Visit the WoW Rookie Guide for links to everything you need to get started as a new player, from how to control your character and camera angles when you're just starting out, to learning how to tank, getting up to speed for heroics and even how to win Tol Barad.