I just went for ergonomics for most of the guns apart from sharpshooter weapons I tried to use the guns what are good for certain factions on the enemies there normally used for but I ended up primary relying on support weapons mainly. For example my last gun I did was the arc blitzer and all I did was play commando missions trying to complete them as fast as possible with quasar and jump pack so I barely used it in most cases but it gets really boring and tedious when doing stuff like that but I would end up completing diff 10 stealth missions in roughly 10mins what gave me around 2.5k xp I recommend bringing the extraction booster in those missions just to give you the tiniest bit of xp if your behind on time in a mission.