It's actually quite easy to defeat. The SSD is fragile in the base game I found.
If you are the Consortium, Skipray Blastboat bomber Squadrons with a few StarViper squadrons to fly cover against any TIEs the SSD launches.
If you are the Rebels, then B Wings and A Wings for cover.
First send all your bombers to attack the Launch bay/Hangar and the engines next. Once those two things are gone the Executor is just one big target to your bombers. It can't run and without it's fighters it can't shoot down your bombers. It's guns are meant for capital ships bot starfighters(similar to how most battleships are vulnerable to aircraft as they were not designed to combat them specifically)
After the hangar and engines are gone target the shield projectors in the middle and rear of the ship(It's dorsal ridge and Bridge tower respectively.)
If the SSD has escorts still focus on the targets I mentioned, after they are gone, then if you have heavier ships jump them in BEHIND the Executor where few guns can be brought to bear(you can do this by clearing the fog of war with a few fighters as they blaze the SSD) then try and pull the SSDs teeth one at a time with your fighters and bombers while your cap ships mop up any support ships they sent.
Do this and victory will be yours.