Becoming a beastmaster takes years of practice, rah, rah, rah... :P Sorry. Bad joke.
Seriously though, being a beastmaster can be awesome. For the most part, I swear by it with my ranger. There are only a few places where I wouldn't take my wolf Omachi.
The Fire Island Chain. Too much degen. If pet's could become infused, it wouldn't be so bad, but they can't. So Spectral Agony just destroys there potential. Also I never used my pet in the Titan areas. I just figured the fire degen would cause too much problems, as I usually count on myself to heal my pet. Monks will not always be inclined to bother.
Some monks would actually start healing my pet half way through a mission. After they see how much damage they do to the monster running them down, I guess they then see them as valuable part's of the team.
Fow and UW are places I wouldn't take my pet either. Though I might experiment in future. I can't say for sure if it would be a complete waste of time or not.
The Canthan mission where you fight Kuunavang. I figured that my pet couldn't hit her anyways (until the end) so there was really no point.
Other than that, I have taken Omachi everywhere. I've even fought Shiro with him tanking. Pet's are very cool for the simple fact that they can dish out some significant damage.
Brutal Strike and Ferocious Strike are the cornerstones of their power. While some builds won't use it for various reasons, most will have them tucked in there somewhere. I won't go into too much detail (there's an excellent pet guide already if you search the main page).
Here are a few of builds I tend to use...
General:
Predator's Pounce
Brutal Strike
Ferocious Strike
Call Of Haste
Predatory Bond
Res
-
-
Degen:
Feral Lunge
Poisonous Bite
Brutal Strike
Ferocious Strike
Predatory Bond
Res
-
-
Usually do THK with this:
Predator Pounce
Brutal Strike
Distracting Shot
Barrage
Predatory Bond
res
-
-
My 'leave me alone' build:
Brutal Strike
Ferocious Strike
Ignite Arrows (or Kindle if scatter will be a problem)
Troll Urgent
Call of Protection/Haste (whatever I think is best really)
res
-
-
Another favourite. Ferocious let's you keep up TF constantly:
Brutal Strike
Ferocious Strike
Read The wind
Tigers Fury (with a vamp short/recurve bow)
Predatory Bond
res
-
-
Well I suppose there's more than just a few. I'm not saying that 'these are the most effective, ftw!!!11" but these are my favs

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