This is the "tie rule" that was introduced in DA.
Since ships always fire back even if they're destroyed now (DA/TA), with the exception of the first round of combat when a civ with the Super Warrior i.e. First Strike ability initiates the attack, it is possible for two fleets to destroy each other.
Since GC2 doesn't support MAD (mutually assured destruction), the tie rule was introduced.
The ship (not sure how it works exactly for fleets) with the highest attack survives the combat with 1 hp.
It's gaining hp due to leveling up by killing your ships, but it should only gain a couple at a time-10 is outrageous, especially for a 22 hp ship.
Since you aren't playing the Arceans or a race with the Super Warrior SA, you have a couple of options.
-You can build ships with sufficient defense to survive the DL ship (75% of his fleet's attack in defense on a single ship-more is better, up to 100%).
-You can build a ship with higher attack than his ship.
-You can have a fleet that he can't destroy in one round. For this, you need logistics, and tiny hulls. At this point, it doesn't matter what weapons they have on them, so long as they each have a weapon. Your goal is to have at least one more tiny ship than he has weapons on his ship-so if he's fielding say 8 black hole generators, you'd need a fleet of 9 tiny ships, or 18 logistics.