This can be boosted with two players but will still take 5-6 hours total (2.5-3 hours for one person to get it)
Important: Set the number of players to ANY. This will allow the match to start with just the two of you. EDIT: This happens unpredictably.
Both play as GDF in a 15-minute Objective Match on Area 22.
One person plays as Field Ops and the other as a different class. The other player can put a rubber band or weight on the RT button and constantly waste ammo while the Field Ops replenishes ammo (0.5 XP per ammo pack).
The Field Ops can also call in Artillery Gun drops every so often for 1 additional XP.
This method takes about 12 minutes for the Field Ops to get up to 84 XP / 3 stars thanks to 30 bonus XP from defending for 12 minutes.
If the other player selected Medic it is possible to get him 3-stars and the Field-Ops 2 Medic stars by boosting for the last 3 minutes.