Getting through all those fighters with the Epee is the hard thing, the turrets are easy with enough Stormfire ammo. Another trick for the Scim is to kill a turret or two and back off to recharge energy. Dumbfires are more useful at longer range, it will fire at you close-up and probably destroy the dumbfire during that process.
Thank for the tip Lazy Panda that worked out, on the second try, cause in the first one I did it and the marines landed, but I fired a FF missile and all the Kilrathi ships turn blue and the missile destroy it and all turn red and kill me, call that bad luck.
I knew that had to happen to somebody at some point - that's really unlucky. Knowing my luck, I'm surprised it never happened to me. Glad to hear you did it in the end though. Have fun flying the Gratha on the next mission.