Get all your soldiering upgrades right away! This includes the Planetary Defense series, Tidal Disruptors, and Tir-Quan Training Center. You may want to take a soldiering ability as well, for good luck. Watching my troops fight the Dread Lords on the ground (or even in space) is quite entertaining, in a macabre sort of way: my guys shoot lasers and missles, they use what appears to be a Crimson Lightning Ray Of Doom that usually takes out at least half of my troops if I have only researched a few of the soldering technologies. Be sure to build farms and upgrade them, because any ground combat will wipe out at least several billion citizens
Considering that the Dread Lords' weakest ship has 100+ in Beams or Missles, you may want to focus on the first-strike offensive tactic first: gather up a fleet and blast the ships to shrapnel before they can strike back. Then destroy any orbiting their home planet and keep up a blockade until you can get your transports in.
The Privateer improvement is absolutely vital on maps with Dread Lords and any substantial amount of trade. They will usually find a way to slip in past your defenses (if you have any), if only to conquer your home planets, and they will obliberate any trade routes they find.
Finally, you can use the baited trap strategy by leaving an outlier planet defenseless, letting them take it, and then recapturing it during the next turn to acquire high-level technology. This will usually devastate the planet used as a trap, because the Dread Lords usually use destructive invasion tactics like Mass Drivers.