Aliens: Fireteam Elite

Best mission to grind this is 2:3, as Synths will keep on spawning until you put the last power core in. But a few runs of this mission will boost your numbers nicely. (with about 50 maintenance synths in one room)

Just a note that killing the synths that are on the Katanka will count for this. so you can get a bout 20 each time you run 1:1 and more when you run 1:2.... not big numbers but if I'd known that I would've knocked this over earlier with all the runs of 1:1 I have done for XP grinding.... :)