I was able to get three (screamer, stalker, wildclaw) last time around with only about two hours of farming total. Screamer dropped pretty quickly. The others not so much. I'm off to get the ronin this time.
I've gotten them all just in the normal course of doing the challenges so I don't think they are particularly rare. At least they don't seem any more rare than other pet drops.
After about 30 minutes of farming spellwrit ronin, I got the pet. The spellwrit ronin are a lot less common that the other spellwrits, though, so it was fewer battles overall than the others. Hope this helps!
