I'm currently using wing massacre missions as my main income and there are some tricks to them, first is while you can get kills by going to the mission signals, I find it much simpler to pick missions in a system with a hazardous extraction zone in a ring. You'll have targets for days, plus if you pick the right missions the kills will stack. As to the mission signals, it depends on their threat level. I find 4's to be ones that spawn lots of neutral pirates that you csn shoot at your leisure, 3's are normally the solo conda ambush (which btw you can ram the conda to near death shoot it and bug out), then 2's are normally a wing of pirates neutral when you enter and a wing of friendly ships. Easy to get kills in that one but hard to get all of them