Under the current system, your rating increase is based on the number of crew in the ships that you destroy, isn't it? I think this represents the best possible compromise, sine the alternative would be to have a very complex scoring system.
To increase your combat rating quickly, in order to qualify for a mission, go to a system where the strands or the UE and Voinians regularly slug it out, and finish off some disabled ships. It's cowardly, but it counts! Voinian Cruisers are your best bet, since they have a very large crew, and are quite common.
In a deluxe scoring system (not possible within EV:O) I'd want a system where kills could be 'shared' between multiple attackers, so you wouldn't take all the credit for the work that your escorts and allies do before you order them to hold position so you can deliver the coup de grace. What constitutes a deadly peril, though? I'd rather face 10 UE Destroyers than 30 Kraits because they would be so much easier to toy with and kill off one at a time. Which victory would do the most harm to the enemy's war effort? Which would bring the most prestige? Any scoring system is necessarily an approximation.
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