Ultranova (Wait, the large transport is simply that powerful?)
But by then it's too late. The gunships have moved the weakest forces back...
And the formation is a circle now.
Plasma cannons and laser turrets open fire with maximum power, superheated matter slamming into the shields of the rebel ships.
(I tested the transport, and against my assault frigate, said frigate caused one hundred forty two thousand credits of damage, whilst sustaining only six thousand.)