Don't use parallels - that garbage gets laggy at times, bootcamp it and partition your harddrive to fit a set amount i.e 80/20 or 70/30 for the windows portion. (Alot of universities and college's out there have free windows using Microsoft DreamSpark) Just make sure you have a big enough flash drive to cover the size of the installation.
If you cannot partition your laptop for some reason, do not try to force partition it. Apparently my mac wasn't able to partition for some reason, so I force partitioned it and destroyed my OS system lawl. I thought parallels worked fine on my mac before it got destroyed. However, Bootcamp -- for sure-- is a better choice.
Bootcamp: Runs Windows and just Windows.
Parallels: Runs Windows inside of OS X.
Bootcamp is the best option. It'll be faster and the only real downside is that you will have to restart your computer to play Maplestory. Perhaps you can say that not being able to use OS X is a downside but honestly I don't really see how it is unless you are creative suite/cloud user. I'm assuming you wouldn't be using Adobe products while playing Maplestory though.
Parallels is nice however it will require your computer more RAM memory than it would to just run one operating system, because you're basically running two operating systems. Which is fine if your computer is good enough for it. Most people would say 8GB is enough for this all (total) though I'd say 16GB would be better. Personally I have 32GB RAM and divide it about 6GB for Windows and the rest for OS X.
My advice, install Windows through Bootcamp and later on install Parallels and make it start up the Bootcamp installation of Windows if you really want to run Maplestory while on OS X. This way you'll have the choice of both.
I have a Macbook Pro that I got this past summer so I think it's the most current model right now with 8 GB RAM and Maplestory runs flawlessly in Parallels. My only complaint would be that I have to press a key every time I want to use an attack instead of just holding it down the whole time. But iirc this can be fixed by getting a bluetooth (or usb) keyboard and using that so it is directly connected to Windows instead of going through OS X. But yeah as for graphics, lag, etc. I've found that it runs better than my old Dell (only had 2 GB of RAM but still) and any d/c's or lag that do occur is never a result of it being used in Parallels and is usually Nexon's fault.
I don't really have much experience with Bootcamp but when I tried it on my old Macbook it froze and when I restarted my computer it had created the partition but I had no way to access it and it also broke the optical drive inside the computer so I don't trust it. And since Parallels works so great I honestly don't think bootcamping would make any difference in-game, at least for me.