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
Ok sorry for the double post but I can not reply so any Mod or Admin please just delete it. I currently know this DOES work, creature levels you listed are natural spawns, but this does work credit farmers have bots do it for Nyax. You may have not noticed because for most planets you need to get the missions from a player city.
So I will now provide an example: Go to Restuss get in a group then leave and go to Nym's Outpost on Lok, get a mission for Gurks(the range will be 88-92 if all members of the group other than you are 90). Now drive to the mission waypoint and spawn the lair then mark it with your own waypoint and delete the mission. Repeat until you have as many as you desire. You can now gain lots of experience on fairly easy high level gurks, some say Gurk Kings spawn I have yet to see it but will keep trying, and from here you can go through your closest waypoinst to get good grinding experience.
This Does Work so instead of listening to SOE(who would do such a thing anyways?) go and try it, even do it with your level 90 to speed up the process just to see that you can spawn higher levels of creatures.
