In order to level up your pet, you must first complete the Mystery of the Five Stones. This requires you to collect all the stones and transfer them to your pet or pets. Once a pet has all five stone on it, you can go to the Elder Council in the Power Temple and Receive the Golden Key.
When you have done this, you must go to the door behind Barkchem and try your luck on the lock. You'll need to gt the five stones in the correct order for the door to open. You only get TEN tries a day, so think it through carefully. If you get the stones in the right order, the door will open and your pet will be able to enter. You will lose whichever stone is in the middle!
For the next part, you'll need to bond a Mini with at least 100 armor to your pet. You can armor your minis with empty bottles and cans once they have their basic armor equipped. This is a time consuming process. You can only add one piece of armor every SIX hours and you MUST have access to Mini City. You should start armoring your mini as early as you can. Each time you go through the door, your mini will lose 100 armor.
Alternatively, You can purchase armored minis from the Mini Trading Post in Mini City. Or, you can have the staff of Everlasting Water and Wind. You can use one or the other or both, but you will lose them afterwards, but this saves your mini from losing armor.
Once your pet is through the door and your mini is armored, the mini will have the option to cross the lava and bring back something to strengthen your pet. This is not an item, it just means that your pet's level will be raised. You will also have a choice of prizes.
The prizes are as follows:
-A three month membership with all the benefits that come with it.
-The Gold Key Fur Tonic. This in NOT an item. It will be given directly to whichever pet went through the door.
-A Skala mini. These can NOT be sold in the Mini Trading Post.
-The Gold Key user look up.