Pirate assasination mission from Sol seems to be okay.
The way it works is pick a system with a few mission – Barnard’s Star worked for me. Then find lots of overlapping massacre missions from different Federation factions.
So Faction 1 gives you “Kill 30 pirates”. Faction 2 Gives you “kill 12 Pirates,” Faction 3 gives you “kill 40 Pirates” etc. As long as they are from different factions then any kills move the kill counter on all the missions.
You can also then add individual assassination missions, these count toward your massacre mission totals and you get the individual mission bonus for the assassination target too.
I picked Sol because there are a lot of Federation Factions and you can fly to Earth and visit the 3 stations next to it to stack as many mission as possible quite quickly.