Generally, when facing a number of powerful enemy ships I found it useful to exploit the stupidity of the AI and attack from a distance. Load up on missiles and slide just barely into range of ONE of the group you are trying to eliminate. Attack this one consistently. If you aren't too close, it takes a while for other ships in an AI battlegroup to respond. And if they don't have long-range weapons it works especially well.
But yes, the Xiati have plasma missiles so they can respond. They are very tough opponents. In this way, you can usually wear down even a Xiati a battlegroup, though. Another approach is to load up on shields and regenerators, and just swoop in to clobber one of them, then run away to recharge. AI ships are usually not well designed and you will be able to recover much quicker than they do.
Re: Difficulty level?
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