You're observing the tie rule in action. Since "destroyed" ships "fire back" in DA/TA, to simulate "simultaneous" combat within GC2's engine, a tie rule was created (sadly, no MAD-mutually assured destruction). The bottom line is that so long as you are bringing ships/fleets to the party that 1) have less attack than he does per ship (which in the case of the DL ships is practically always true) or 2) aren't going to survive the first round of combat in the first place due to the other change that DA/TA brought to combat (each weapon fires individually and as such a ship can take out as many ships as it has weapons per round), you're not going to kill it.
You can get lucky by not meeting 1) while barely meeting 2) but since the targeting priority is a function of attack divided by the total of defense and hp, if you're using the same size of hulls at least, your high attack ship will be the first to go down regardless. Still, a mix strategy can be done if neither 1) nor 2) will accomplish it, by soaking up just enough damage with larger ships, after all the smaller ones pop-this is somewhat dependent on the DL's current firepower and more specifically their weapons layout (say 9x nightmare torpedo on a ship).
But it's not impossible, just really difficult if you let them live long enough to build huge hulled ships.
The reason it has 8 hp after the battle is because it leveled up, by the way.
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