Myself, doing that (and adjusting the experience gathering to get the original level on its 0-20%!), I have 600+ universal training stones and two unused ultimate (purple) ones.
You can also keep in mind that if you level your pets on a non-max level character, that toon gets experience as long as your best pet is up to 5 levels above the battled ones.
The easiest way is just battling the 25s with a levelling pet and it also gives the best results for a player. Not very encouraging to visit the old areas!
