How do I destroy the Sathanas' beam turrets?

Step 1) Call for a rearm ship right away so it follows you around for reload.

Step 2) Fly at the Sathanas so it is moving towards you, however stay on the outside of the ship, flying to the front-centre will get you hit with the flak guns.

Step 3) A double Helios shot will take out the turrets, or bring them low. Follow up with your Trebs

Step 4) Fly away from the Sathanas towards the jump node, turn around, do it again.

Step 5) Stop, let the rearm ship refuel you, when it is done call for a rearm again just in case.

Step 6) Keep doing this over and over, you should be able to take out all four. However, only two are required to complete the mission.

Step 7) You'll be asked to destroy extra turrets on the Sathanas, this doesn't carry over to the next mission, so instead just get clear of the Sathanas so you don't get hit when it jumps.

After that, rearm again as a Demon-class destroyer comes in to play, I'll let the mission tell you the rest.

If I remember correctly, those turrets require double helios missles to kill. What you should do is dive straight at it with afterburners on, shoot off double missiles, then pull out. Before making a second run, make a call for a resupply ship as you make a run at the second turret. After taking care of that one, immediately resupply (as the ship should be following you at that point), and attack the last two turrets.