Alright mate,
Which primaries are you using to defang those SOBs, if at all? If you haven't worked out the piece of win that is the Archer, I'd suggest having a go with that baby. It doesn't penetrate shields, but it's as close to a turret sniping primary weapon as you're probably going to get in Freespace.
If you can attempt to neutralise the turrets before the beam overcharge point, then bravo-zulu. But basically, as soon as the sequence starts, you need to prioritize the closest corvette, and neutralise his forward beams and aft beam cannon that has a line of fire to the frigates.
If memory serves correctly, he'll attempt a roll on the x axis, use this time to neutralise the cannons of the other Corv with the Archer. (You can do this at standoff range, cruising within the safety of the corvs). Now, once you're done that, the first corvette you disarmed will be bringing his other beams to bear, so at this point you want to finish him off, and then get back to that corv you left a minute ago and destroy the rest of his beam cannons. (K is a handy little key for this segment)
This can effectively be pulled off in under thirty seconds (and should be).
If your frigs are being mauled by the other ships in the Carthage's group, then don't hesitate to defang any single ship causing damage (I think G is the key to hit whilst targeting a ship to target its current attacker?).
What you also might want to do, is try and neutralise the bombers in the first phase of the mission as fast as you can, and keep both Frigs in the 70-80 range in terms of their hull integrity. This'll act as a buffer when it comes to the beam overcharge sequence.
But of course, I've played this mission literally about 20 times and have that whole sequence mapped out in my head and might have put it down in a post in such a state that it's unfollowable, in which case I apologize.
