Probably prismatics I'd guess but depends how you've engineered your weapons if they were tough to get through.Myself and some squad mates have gone into a couple of these Pirate Threat Level 7 sites and engaged some of the pirates. We had brought 2 fully engineered Federal Corvettes, 1 fully engineered Anaconda and 1 fully engineered Krait Mk 2.
We targeted and engaged these pirate ships one at a time, and we choose to attack each ship singley. Meaning we choose to NOT attack pirates that were in a wing
We chose one target, then closed to a range of about 1km then all fired at once. It took almost a full minute and a half of constant Multicannon, Laser fire to cut thru their shields and almost another 2 min of firing to bring their hull to zero
I don't know what they had for shields but it was more powerful than what I have seen in this game. Where do I get those kind of shields.
The other thing to watch for is chaff throwing off gimballed weapons. Dropping target lock while the chaff is dropping can help if you can stay on target but it takes practice.
The other that helps is aulticannon with corrosive rounds to help cut through armour once those shields are dropped.
I have a corvette heavily engineered along those lines and was having decent success in a threat 5 solo other than a buddy in my fighter last night.