No, no, no. Just... no.
The WinSxS folder (also known as the Windows side-by-side assembly folder) is required if you want your server to work.
If you want to try to reduce its size, you can run a cleanup on it, but I'm very concerned for your data. A dozen GB is not big - and having lots of data is all the more reason to back something up, not an excuse to exclude it. Losing more data is worse than losing less data, after all.
If you fail to backup this directory and need to restore your server, you won't end up with a working server after you're done. Just back it up, and buy an extra LTO tape/external hard drive/whatever you put your backups on. They're cheap enough, and certainly cheaper than having a bricked server.