Game is wonderful.
you can play it with medium ish settings in OSX it seems. It's still in beta so improvements are possible. Forum here;
https://forum-en.guildwars2.com/forum/support/mac
good luck mate! If you really. If you love the game so much that you want that high settings experience, on bootcamp, then you could make 100 gb partition.
Buy an external USB3 hard drive and put on all your songs, itunes library, movies and other stuff that takes space. It can easily work out.
Maybe in a years time (or two) you could do an aftermarked SSD upgrade, and get that 480 gb SSD upgrade;
http://www.anandtech.com/show/6154/...d-upgrade-for-macbook-pro-with-retina-display
Its still very expensive, but who knows. In a couple of years it might not be and then you might get more mileage out of the machine. In this day and age 256 GB storage is very low. SSD prices just havent moved fast enough.