Is this your filter: http://www.pentairpool.com/pdfs/TitanRPMOM.pdf ?
If so, there was originally a special bulk head wrench: http://www.poolplaza.com/P-AMP-051-1061-ATC.html
But, it seems to no longer be available. It appears that a complete replacement bulkhead is available: http://www.poolplaza.com/C-American-Titan-SS-DE-Filter-Parts.html
You may have to destroy the existing bulkhead to replace it. I'm not familiar enough with that filter to know if there's an alternative method. But if you post a picture with Webshots, Flickr, etc, I'll take a look. (If you don't know how, email pictures to
[email protected])