Go to the Machine archivist in Midian and buy the 4.2 archived critical missions. Complete the first 3 missions and in the third phase of the fourth mission, you will have to meet with Agent Pace. She's going to tell you that you'll have to get rid of Mammon, the leader of the Daemon Army mob and she will give you two bodyguards - two lvl255 Agents.
Well, these agents will protect you from every and any NPCs attacking you. If you're low lvl, this is the ultimate grinding mechanism for you, as you actually receive XP for the kills of the agents.
Also, if you're higher lvl, you can take the agents to the more populated areas of Westview and attack mobs, the two agents kills anyone attacking you, so you shouldn't worry. In about 1 hour, I have managed to farm countless red frags and about 5.5mil $i. Just start shooting at the mobs as soon as they shoot back, they are dead, the agents do one shot, one kill.
Oh, obviously, you can not use HL's, so you would have to walk everywhere, but it worths the effort. Also, note that the mission times out after about two hours and you will have to wait about 5-10 minutes to be able to start it again. The mission can only be started in Richland (or in Westview, but you'll be sent to Richland anyway)....
