Remove all layouts. A new layout will be created. Make sure there is no viewport. **REQUIRED**
Delete all unneeded components. Verify by zooming to extents.
Purge all multiple times until it will remove no more objects. Typically 3 times.
Purge Regapps
Remove all hatching, not required but good practice.
Flatten
Overkill
Create a new Revit project. Import AutoCAD. Explode all. Convert AutoCAD to Revit objects.
Purge Revit file.
Import converted native Revit objects to the new file.
DO NOT explode AutoCAD imports in Revit production files. AutoCAD files are only to be exploded when converting to Revit objects in an exterior file to be linked in (NEVER IN A PRODUCTION FILE)