We will do our best to fix this, but until then yes – you cannot lose focus from L2.exe window or it’s going to crash (doesn’t matter if you’re full screen, windowed mode, if you use alt-tab or windows button). It’s the fact that the window loses focus that makes it crash.
I know this is really annoying and we’re doing our best. I have noticed that running TeamViewer (justs launching it, not connecting anywhere) makes L2.exe stable as a rock, I can do my work properly and have L2 running in the background. I know it’s a silly fix but it’s a fix for now, especially if you are using TeamViewer (TeamViewer is official software, all you have to do is download it and let it run in the background – it won’t do anything to your computer, you also don’t need to use it in any way. Just let it run there).
EDIT: You need to create a TeamViewer account and sign in (on the right hand window, “Computers & Contacts). Also try disabling Desktop composition for L2: http://i.imgur.com/6gOhkIZ.png