It's a grind. You have to unlock each safe house then do the side missions for that zone. When you complete the 3 in one area another unlocks and gives you some bonus xp. I went a different route as grinding was boring for me and match made the higher level missions. It was tough but I was support class so I wouldn't be as much of a hindrance to the team. Check your gear drops too. If xp is your focus try to keep stuff that gives a boost even if you take a hit in stamina or dps.