322m2, 5.09km/s, 732kc, 411t. Two-thirds fluorine by mass. A 28-man laser can which actually features the finest laser I've ever designed, with a minimum-length rod costing 183c. Making the mirror (the one just called 'mirror') aluminium instead of silver brings the cost and mass down a lot. I initially tried copper mirrors, inspiring the name. Despite this 'amazing' weapon, the ship probably would be better off with conventional cannons. The flares are the main defence against missiles, though only against conventional warheads; nukes going off anywhere near it will make it implode in shame.
The RTG powerplant (13.6kc) is cheaper than the crew overhead of having a reactor (40kc). Though once you do bring reactors you can stick more efficient thermal rockets on so I don't know if it makes the ship cheaper overall. Given how disgustingly cheap fluorine is, though, it's quite possible. The low heat output synergises well with flares in any case, which when loaded into blast launchers don't require any crew at all.
The missiles are the primary weapon. A simple derivative of my Helix model with 300 x 1g lead fragments. Coming from one of my more realistic designs, it does have a hypergolic motor, so it's not as meta as it could be.
You may want to change change the score equation. Stock ships are already very easy to kill, and the lack of a required AI mode makes it even easier. You can easily kill 5 Gc worth of stock ships in defensive mode with a railgun skiff.
Andromeda Drive Yards presents the PT-4 Hornet II class patrol boat. Like its predecessor, it is armed with 1,200 micro-missiles and is intended to fire at standoff range, negating the need for heavy armor. The sheer volume of missiles it can fire has also shown to be confusing to enemy ordnance, making anti-missile countermeasures unnecessary.
It killed two gunships without issue, and could probably kill two more.