Here's my method, which works fairly well: Max out beast mastery, and then put the rest into WS or whatever (for a self-heal). I also recommend equiping a weak spear (the one from the Kamadan weapons crafter, for example)
Use [skill]Call of Protection[/skill][skill]Call of Haste[/skill][skill]Symbiotic Bond[/skill], as well as the obvious ([skill]Charm Animal[/skill][skill]Comfort Animal[/skill]). Use the remaining three slots for a self-heal and whatever beast mastery skills you have (pet attacks work well).
Anyway, go out onto Plains of Jarin, and activate those skills I mentioned. Attack targets with your spear once (so your pet attacks), and then stop so the pet does all the fighting. The vast majority of monsters will attack you instead of your pet (for reasons I never did understand). Pretty much any monster level 6 or higher will do, really. Anyway, once your pet reaches level 8, take a little trip to the Desolation (access to wurms is necessary, access to Bone Palace, Lair of the Forgotten, and Wurm Siege help too)...
Anyway, take a hero/hench party to areas of the desolation with large stretches of sand/sulfur (The Alkali Pan works very well for this), get into wurms and kill everything that moves (this is a good time to cap skills and open chests, btw). Repeat until your pet reaches level 11, and see what evolution it is. If it's Aggressive, keep it up until your pet reaches level 15. It should (but won't always) turn out Dire...