Packhounds hit them and "may" reduce the number of Tharglets in a swarm (as does ramming the cloud with full shields) - very hard to tell as you can't just sit still and count - they also seem to take evasive maneuvers and try and out run the packhounds - this seperates the "swarm" from the main ship and can take aggro of you for a bit.
Damaging the mothership witrh 3 or more salvos of packhounds seems to make it more aggresive and change to using missiles itself, as well as what to me looks either like a burst laser or maybe some sort of rails \pulse weapon with a "trail" on the projectile - in the pre-release video the Thargoid "healed", perhaps getting it's health to a certain point "triggers" different behaviour in combat tactics ?
My PD was definately taking out incoming missiles as long as I dictated the range - Python with grade 5DD can out turn and outrun "one" Thargoid - would be intrerested to see how an engineered gunship with fighter might fare (the NPC Fed ones are all wrecks mind

).
I am also reminded in the original game they tended to come in three's and worked well together - this could prove tricky.