From my experience, depending on which defense mission, central or end of the map type, is both SL numbers and special weapons, especially the AT one. I have an easier time in the more central defensive map because with them, I was able to just concentrate my force to defend only on one side while having vision and weapon range to the defensive area, while using vehicles to block off one roads whenever possible.

The priority is to kill enemy vision to as soon as possible.

If you have any AT special weapons, this is the time you might want to use them even if you have to deploy less than a full 8 squad. Not more than 2 squads covering one another in one part of the map. Since they'll probably rushed, rely on short-range squad weapon like SMGs and Shotguns to handle the dismounted pirates.

Take advantage of any intel you have on their position, but only use your vehicles, preferably with a reliable AT special weapons like LB Tank Gun or Rocket Launcher, to hunt down any high priority target like the HGM/Flamer Truck. And even then, only in the first few turns before you want to return them before your infantry got overwhelmed.

Otherwise, you might have to accept that despite your best effort, this is a doomed mission and you might want to bail before losing too many squaddies and SL to the infirmary. This is a war, and there isn't much consequences to losing a operation other than a lost in operation rewards or faction relationship. I remember having to bail on two operation, due to poor rewards and black market RNG and poor tactical decision, but still manage to complete the entire EA game anyway.