Missions are great for exp. I have 3 friends that have been playing and I just started back. With the double exp that is going on this week I was able to get over 70k exp in all my ranges attacks within an hour each. Each member of the group grabs 2 missions in the same direction and then you go out and slaughter till they are all done. Rinse and repeat and you will be rocking! Also, the missions outside of cities (i.e. in player cities) tend to give a much higher reward for the missions. I was excited when I was getting over 1k credits in the cities before I met up with my frieds and we were all getting over 8k per mission with the ones in the player cities.