I conquered that pesky mission last night all by myself using the Long Range Rokh and flying out to a distance of 150 km and then just plugging the rats as they flew towards me. Surprisingly quick and easy, I did it in one go. Such a relief.
I figure my initial problem is I was trying to use the Ferox and then the Rokh as a club and beat the rats into submission because that is what worked so well before. But both of those ships have range bonuses and once I used the Rokh with range enhancing mods it was easy to do the mission. I'm sure the same tactic on the Ferox could have applied as well.
Now that I'm more experienced and wiser, hopefully next time a similar mission won't give me as much trouble.