The real threats are TRN Epsilon. There are two waves of four TRNs which will try to disable the Ars Opus. You must destroy them or the rescue operation will probably be a failure. Target a TRN and use two missiles to lower its shields and then lasers to finish it. Be careful not to follow too closely or you may run into them and that will be fatal. Remember too, you can't use the 'R' key to target a TRN, you'll have to use the 'T' key to cycle through the targets to find a TRN to attack. You should try to ignore the Y-Wings as much as possible until the TRNs are dealt with.