This is probably old news to most members:
Pets can gain a great deal of combat experience quickly when you have them attack the Crazed Mage located in Shame Level 4. The mage doesn't do a lot of physical damage but is tough and hard to kill. It is similar to sparring with a Shadow Iron Elemental, however the combat experience spools up much faster.
When training lower level pets, I use a tank Cu to attack the mage and then have the lower level pet attack second.
Most pets will begin to display the message, "Your pet no longer gains experience" after a certain point. For example, I trained a 4 slot GD from 10-80 in about 45 minutes today. I used 100+ bandages to keep the GD healthy.
I marked some runes to this area and left them in a rune book in the guild chests is anyone wants to try this. Here's a screen shot of the area: