Hey bro! I just got Arcturis myself, and with all the competition because everybody and their dog knows where he is, he can be a toughie. I was lucky enough to tame mine while the two other hunters were pvping eachother and the spawn killing ganker was afk ^__^
But enough about me- After about 4 days of hard, grueling camping I figured out his pattern and nuances. I just decided to share because well, we're all fellow hunters aren't we?
I deduced from the internet and my own timing that he runs a 6-12 hour spawn timer, and rotates said timers in an erratic fashion, such as
12hr-12hr-6hr-12hr-6hr. What I did was hard camp him when I could, and checked at his spawn timers when I couldn't. I was never disappointed, and after 5 failed attempts I had it down pat

Once you learn the last time he'd been killed/tamed you can get a decent estimate of when he'll pop back up.
Also, he never spawns down by the waterbank. A lot of people thought he did and camped down there, but he spawns directly on the hill, in the middle ground between road and waterbank. Stay towards the middle!
I made a macro for when he popped up, because at one point my panic for button finding to tame him killed him. My macro was
/cleartarget
/target Arcturis
/cast Tame Beast
And it caught me my bear! Also, NPCScan is a MUST for this pet. As well as Glyph of Tame Beast, to lower your risk of a ganker killing the bear before your tame.
Well, my ramble is over. I just wanted to give a fellow hunter some tips that helped me get my bear. And if I'm ever on when you are, I'll invite you to my realm for a quick look!