I'm afraid it simply is almost impossible to do it without cheating. I used to be a real WC1 maniac back in the days and even then I only managed to beat this mission ONCE. I still don't know how I did that; I guess I was extremely lucky with my shooting. But otherwise, it's useless. The same thing that makes a Ralari an easy target for you (laughably low hitpoints), makes it an even harder thing to defend.
I guess you could try using taunts. If (and that's a BIG if - I haven't played the game for a loooooong time, I'm not even sure if it's possible to pull off this level of enemy management) you can divert three, or at least two of the grathas from attacking the destroyer, it should buy you enough time to take out the one or two that ignore your mating calls, and then you can finish off the ones that do turn their attention to you.
The good news is, you don't need to beat this mission. As long as you succeed in the other two sorties with Bossman (rather easy), the game will consider this series of missions a success, even if the Ralari is destroyed.
Post edited January 15, 2013 by Ardis