Do the explore missions that are target assassinations. I forget the symbol, I think it is the rounded triangle one with the star in the middle. Trade in spinmetal for marks and just keep farming for rep. Oh, also get a group and have them split up over the entire map. That way it's not a long ass haul back and forth.

So do you just run around until you find an assassination mission, ignoring the rest?

No. Spreading out works well. Prioritize assassination, then do the rest. Sometimes when you complete a couple missions the other ones in a zone get cycled or recycled. Just prioritize assassinations, then the kill x units/collect x from units, then the scouting ones. Always look for them in this order once you complete a mission, and allow the closest player to pursue it while you stay in your area. You'll have to learn what the symbols mean. They aren't hard but I keep forgetting.

So you try and keep the group centered in one area? If yes, which area did you prefer?

No, the opposite. The second area + the old snowy place where the strike mission is is a good area for a player or two. Then the Skywatch area where that ?? Hive guy is over the cliff edge. Then the Forgotten Shore. And another in the very beginning. I haven't done this a whole lot but these are good areas for the beacons to spawn. Although, there was certainly a preferred area for me - that second area where the downed planes are and it is easily accessible to the starting spawn where a very easy fallen boss spawns, that one story building where another boss spawns, and has access to all the missions from the starting place, the second area, and the aforementioned area with the snow on your way to the strike mission.