I'm doing Render Unto Caesar. NCR gave their 3-day warning and then sent Rangers. The first fight was pretty funny, I got attacked at Grub n' Gulp, the proprietors got involved, the brahmin got involved, and I ran into a herd of other bovines, all trying to kill me. But I didn't get the Rangers, and I didn't know where they went. Then as I was traveling through the same area, I was attacked by the Rangers. After an unpleasant slogging match, I finally killed them. But then I got attacked again outside of Novac! I really don't want to fight a new group every hour, how can I avoid it other than always fast-traveling?

Every time i exit a house, a factory, etc. There's a legion assassin party waiting for me. You just have to get used to it.

There are only two ways to stop this. Get some good reputation with the NCR(which consequently may be impossible that late in the game or may incur the wrath of Caesar and Legion assassins will start attacking you instead) or finish the game.

Faction armor is a trick that works for Legion Assassins so it may work for the Ranger hit squads as well. With Legion if you put on NCR armor you're still kill on sight to the regular Legion you meet but the special Legion Assassins don't spawn. Just be careful if you do anything to change your reputation while wearing the armor as your faction will change. Just remove and replace the armor to reset to base line.