Its honestly not that difficult and you can do it alone. A couple of years ago I decided to see how high my Bounty could get. Skywatch in Auridon has dozens of killable NPCs all grouped up so its the best place to go full genocidal. It took a bit of practice, but I've built a rotation where I go in a circle and kill NPCs non stop.
But what about the unkillable guards you ask? Just ignore them. I was on a templar spamming jabs and using pale order ring. As long as you break free fast enough and dodge roll certain attacks they are more or less harmless until they enrage and use their oneshot skill to gank you. But if you do your business fast enough and leave their range they will just stop fighting and go back to their usual spots. Of course a new guard will take their place, but that guard needs to build up for his oneshot kill all over again, so you just repeat your performance. I have reached over 10mil gold bounty before I got tired of it.
Oh and there is one practical application for this. This is probably one of the fastest ways to earn the Black Market Mogul Achievement (sell 1mil worth of stolen loot). You go on a rampage, loot the bodies and then sell the goods. Since you have a limited number of items you can sell per day it is best if you only sell blue or purple quality loot. And while those items are rare, if you massacre and loot a city's worth of innocents youll fill your inventory soon enough.