Holy crap was this mission frustrating!
Managed to complete it today at last. Don't know exact number of attempts it took me but probably between 20-30.
What worked for me:
1. Set guns to Full, missiles to Spiculum
2. Autopilot to asteroid field, and ignore Salthi
3. Tell wingman to "Form on my wing"
4. Aim for Ralari but don't autopilot, just tap afterburner to get there
5. Around 15,000km from Nav 1, the Ralari will appear on screen. Tell wingman to "Break and Attack"
6. Shortly after the Gratha will appear on screen. There is one that is closest to the Ralari coming in (approaching at 2 o'clock). Aim for that one and afterburn to get near it ASAP!
7. When Spiculum locks on, launch it. Once close, spam the other missiles (but save a couple) and shoot guns. My aim is to destroy this one asap
8. Aim for any other Gratha shooting at the Ralari and again kill it quickly
9. Once there are only 2 Gratha left, it becomes significantly easier since the other wingmen are keeping them occupied.
When I tried this strategy for the first time, I noticed it gave me a little bit more time before the Ralari was destroyed so I stuck with it and beat the mission after maybe 5 tries. Of course a whole lot of luck is involved!