SQL 2008 R2 Datafile Too Large To Empty or Remove
Shot in the dark without any information on how the database is structured i.e. what tables are on what filegroups, is there an existing partition scheme?. 

How about rebuild the tables on the big filegroup to one of the other existing filegroups, or a new smaller one?

CREATE UNIQUE CLUSTERED INDEX PK_MyTable ON dbo.MyTable
(
&lt;column list&gt; 
)
WITH (DROP_EXISTING=ON, ONLINE=ON)
ON &lt;new filegroup&gt;