This is fairly simple but I've noticed many players do not know about this.
When taking a mission(player cities or outposts) the mission will be for your level or group's level. So you can set the level of what you want to grind by getting missinos spawning the layer deleting the mission repeat until you have plenty for quick grinding with an exp buff.
There is more though, you can pick if you want to fight gurks or Nyax or anything that might drop something useful or worth some credits. Also you can pick something that gives slightly more experience with lots of ham or you can pick easy kills and speed through it whatever to you seems best.
So go out and grind up a storm and level up
