As far as I know, there is no way to do a backward restore, even if the database was in a compatibility mode that matched the lower version. The compatibility mode impacts how certain sql constructs behave on a database but it doesn't mean that the database is actually internally structured as a database on that native version would be. A 2012 database, no matter what the compatibility mode, is still a 2012 database internally, so a 2008 server wouldn't understand that internal structure or be able to restore it.
Best advice is to script out all the objects, create them on the older database, then copy the data.
And then again, I might be wrong ...