I tried to explain this earlier in the "easy way to take down a Sathanas" thread but it was locked, so I want to explain something I've discovered. I call it the "Doorstop Strategy". First you take a capship (corvette, usually Sobek, works best) and place it right in front of a Sathanas, so close that it's actually in the space between the "arms". It will be too close for the BFReds to target, and the Sathanas will be unable to turn without colliding with it, which will keep it stuck in place until the corvette is destroyed (hence the "doorstop" name). This leaves the Sathanas open to attacks by capships on its other flanks!
This takes a lot of trial and error to set up, but it does work.