If you are alone with Ragnar then you should be ready to war with him. If you're on a map with other AI, he can be bribed to backstab others at cautious, which is pretty beneficial to you diplomatically. It's it's especially good if there's a protective AI who unitspams, like Sitting Bull or Charlie or Toku because Rags will bang his head against the wall attacking him, causing both of them to research poorly. It's good too if you can bribe him against good researchers like Darius or Pacal to slow them down, just make sure Raggy doesn't capitulate them. The same strategy can be used with many of the crazy war monger AIs like Shaka or Aleexander or Monty. Use them to your advantage, and when you have a tech lead swoop in and take some cities.