Although LibX has a powerful to unpack Xenocode, I would like to introduce another way to unpack Xenocode with OllyDbg. It is not an optimal way because with it you can not get all assemblies back, but it will help in some cases.
Use Internet Explorer to open .SWF files. Press F11 to have a full screen.
Xenocode Postbuild 2007: http://hintdesk.com/Web/Tut/Reverse/DotNet/Xenocode.zip
Xenocode Postbuild 2008: http://www.mediafire.com/?iz5dzltttfm