If they have beam defenses, go with either of the other options. Missiles are generally a better option, but guns will work.
If you played Dread Lords, you beat them in that campaign with simple firepower. The combat mechanics for Dark Avatar and Twilight are somewhat different - defending ships get to fire back, even if they are destroyed that round. A round where both fleets get destroyed is settled by the tie rule - the last surviving ship of each fleet is compared, and the one with the higher attack value survives with 1 hp. To beat a Dread Lord now, you must get arround the tie rule. There are basically three ways to go about this:
1) Fighter swarm. Get your logistics as high as possible, then cram a fleet full of the best tiny hulled ships you can build. The Dread Lords can only kill as many fighters as they have weapons each round, so some will survive to fire a second round. This tends to be hard on the fighters, but they're cheap.
2) Defense. Research up the defense to whatever the Dread Lords are using, the get as many medium hulls as possible into a fleet. Each one should have a decent weapon and as much defense as you can get on it. It's the same principle as the fighter swarm, hoping your ships can tank more hits than the Dread Lords can dish out.
3) The Glass Cannon. AKA the bigger hammer. Research up to at least nano rippers, two or three levels of miniaturization, and huge hulls. Engines are recommended, but not required. Design a ship with the maximum firepower you can get on it - make sure it's well above what the Dread Lords have on any one ship. Then rush build it on whatever planet's nearest - this will likely bankrupt your empire, but this one ship will win the game.
