All you do is go to the desert and go where the camel is and wait. In a few moments two zombies will show up. As long as you stay in the area the zombies will keep coming. This is how I got my monsters upgraded!
First make sure you have 2 Rheebuses, a Decoy Pillar, a Carbuncle, and a Hell Hound in your deck. The other 25 cards should be cards that you want experience for. Enter Fossil Boneyard. Also make sure you have already completed this level. Get your Hell Hound out and go to the place where you fought the Amber Dragon. You should see 3 caves of of Baby Dragons nests. In the middle of them place a Rheebus down. Then place another Rheebus next to it, so they heal each other. Then Z-effect the Decoy Pillar, and Carbuncle. It should turn out to be a combo. Place the combo in the middle of the two Rheebuses. The Baby Dragons should be attracted to the Decoy Pillar. Since everything heals, it is everlasting! You don't have to do anything! If you want, you can get the crystals for money. This trick should give you about 30,000 experience points in 30 min.
To accomplish this trick, you must have room for at least two more files besdies your main one on the same memory card.
Start a new game, abort the first mission, and save your progress in a new file. Now go to the VS mode and load your main game (the one you want with the cards you want to copy) for Player 1, and your new game for Player 2. Wager the cards that you want to duplicate, then lose to Player 2. Load your new game, then save that game in however many slots you want to make that many copies of the card(s) that you want. (don't overwrite your primary game!) Go back to VS mode and load your main game as Player 1, and the copy new game as Player 2. Wager the cards that you have to copy, then win as player 1 and you'll get back your cards. Go through all the duplicate new games, to receive all of those copied cards. This works with any card including rare ones such as the Golden Goose and Capture Card.
For your deck, you need 1 Claws R Us, 2 Rheebuses, and 27 cards you want exp for. Go to the fossil boneyard, and discard until you have the Claws R Us and Rheebuses in your hand. Power up the Claws R Us and toss it right in front of the baby dragons' cave. It should start killing them with a single hit.
Next, toss the Rheebuses so that they're right next to eachother and the Claws R Us. They should be healing each other and the Claws R Us. Now, go do something else for a while and let it sit. After a while, come back, and you will have gained a ton of exp with next to no work.
In Ruldo Forest, go to where Gurd was standing and walk through two trees there you will face three Cocktraces. Defeat them to have access to the battle arena in the mountains. There you must defeat all guardians(if you cannot go in, improve your skill level with that element until it has six stars) and collect there cards to unlock the second arena. Raise your ability with the Neutral element to six stars and go through the door. You will face a really strong creature that guards the next door. Defeat it and proceed. In that door, you must defeat Helena. Her stratigy and cards are very similar to the one she used in the first Lost Kingdoms. If you are victorious, you will recieve an Ice Skeleton card and you can now choose Helena as one of your characters in Vs Mode.