For DL, it is pretty simple.

First, ships are targeted based on the result of (attack power / (HP + defense)). The highest result is shot at first.

For each round of combat in the fight, each ship in the fleet fires in turn (same order as targeting, IIRC). They bring all weapons to bear on highest priority living target, ignoring any potential for overkill (example, the enemy has 1 HP and your ship has 20 doom rays...). After all ships have fired, those that were reduced to 0 HP or below are removed, and the next round proceeds if both sides still have at least one ship.