Most people think that Loque’nahak is the most difficult spirit beast to tame, but I’d argue that honor goes to Arcturis. At least Loque has multiple spawn points, so you could always get lucky and find him at one of the spawns where no one is currently camping. Arcturis, on the other hand, spawns at a single location and almost always has at least 1-2 hunters camping that spawn point.
One of the problems with this is hunters will fight over the spawn and Arcturis ends up dead in the process. If you’re taking the camping approach, it might be best to befriend any other hunters who are camping. Perhaps you can come to some sort of agreement where neither of you will be jerks and kill the bear after someone starts taming it. If that doesn’t work, you might need to become a little more dedicated.