How does one get invulnerable ships?
So I started the first campaign in version 2.02. I am on the mission where one needs to capture Teth. So I built up starbases and fleets of sacrifical ships to take on the Dread Lords' vessels. But these vessels never die. I can get them to 1 hit point (costs me 3-4 phoenixes to get there, each has a 5 firepower and +4 from star bases). After that, no matter whether I send 1 ship or a fleet of 3, I cannot score another hit. The level of the Dread Lords ship does not change and its hit points stay at 1/22, but often it even gets to 10/22 (once again with a level bar midway to level 1). Are their vessels vampiric (but if so then why only when they are down to 1 hit point)? Why is it always the bad guy who has the invulnerable ships? I want some too...
A way to win against an invulnerable opponent does not appear to be straightforward to me... Any suggestions?