If you have access to a monk hero, just equip her with resurrect and orison of healing. Put her on avoid combat and disable orison. Find a pet, take off all armor, then hit it once to aggro it. Flag your monk at the edge of your aggro bubble. Now the pet will kill you, the monk will rez you, and since she tries to avoid the pet and since you have no armor, the pet will go for you, kill you, monk will rez, repeat... Go watch TV for half an hour. When you get back the pet will have leveled up. If not level 20 yet, wait a while more.
When the pet reaches level 20, put on all armor as soon as the monk rezzes you, then enable orison of healing so she'll keep you both alive. Then just charm your new maxed out dire pet.
If you don't have access to heros, you can always go kill the minotaurs outside Ice Tooth Cave or something. It doesnt matter if your pet dies, she'll still get xp (she won't level up until she's alive again, but then she might gain several levels at once). In fact you can let the pet die, then clear out all of Anvil Rock if you want (which should not be difficult for a level 20 W/R).
