To get experience early in the game, complete the first two stages (the starting castle plus the training stage). You should now gain access to the Apothecary and Blessoon. Go to the Apothecary and talk to Guru. After speaking with her, Dahl-Nok Valley will become available. Go to Dahl-Nok Valley and go to the right. There should be bee hives there. Select them to fight the Giant Bees. To kill them easily, send out your Man Trap. Have it attack the Bee once, then call it back. Finish off the bee with any desired character to gain the experience. Note: Bees cannot be captured. Each time you kill a Bee, you should gain about 100 to 140 experience. After killing about three Bees, return to the main map then go back and repeat until you get the desired amount of experience.
A very easy way to gain money early in the game is to go to Blessoon as soon as it becomes available. Catching most of the creatures and selling them will give you more than enough money. The easiest way to get money is to catch and sell the Rats. You will get about 40 gold per rat sold to the Apothecary. The most expensive card, the Guro only costs about 210 gold.
An easy way to get an extremely rare and strong card early is to use Lizardman. Get him 6000 experience then transform him into the Red Dragon. The Red Dragon is extremely strong, but may cause you to lose HP early in the game since you have a inexperienced character. Another rare card to get early in the game is the Hydra. In order to get the Hyrda, you need to use your Man Trap and gain sufficient experience to turn it into the Catoblepas. Next, use the Catoblepas and get sufficient experience (about 2000) and it can transform into the Hydra.
If you see a powerful enemy that you like, get its life down to about 2 or 3 then hold L and either A, B, Y, or X to capture it in a card. Note: Use a weaker card so you do not waste any good ones.
Take a very powerful summon card, such as a Red Dragon or a White Tiger. Put it in a small deck including two or three copies of the card, and four Sea Monks. Go to a difficult level. Use the card, restore it with a Sea Monk, then use it again, then use another Sea Monk. It should restore the Sea Monk you already used and the powerful card. This allows you to use the same card repeatedly, even if you only have one of it.
This trick requires two memory cards and controllers. Save a file (the one you want to keep) on a memory card. Next, on a second memory card, create a file that is temporary. Get considerably far or as far as you would like. When you get a card you really like and you cannot get more of them because of lack of experience, copy your file that you want to keep on the first memory card as many times as desired. Then, go to two player mode. Place your first copied file as player one and the temporary file (on the second memory card) as player two. Bet the cards you want to copy from player one, then lose the match. Switch to the file that is (non-copied on the first memory card) and have player two bet the cards that they just won. Win the match for player one. Repeat this as many times as desired. To copy a file, go to single player and save under open slots.
Assume you have a Red Dragon card that is to be duplicated. First, in your main game (with the Red Dragon), save it in two separate files (hereafter know as 1A and 1B). Then, start a new game, and name the character with a different name than in 1A and 1B. Save it in a separate file (known as 2). Start a multi-player game. For player one, load save 1B. For player two, load save 2. Bet the Red Dragon in player 1B's possession. Then, have player 1B intentionally lose to player two, so that player two gets the Red Dragon Card. Then, start a new multi-player game and have player one load save 1A and player two load save 2. Then, have player two bet the Red Dragon card. Have player two intentionally lose to player 1A, so that player 1A wins the Red Dragon. Now, go to one player mode and load save 1A. Player 1A now has the Red Dragon card. To do this trick multiple times, while in one player mode have player 1A save the game in save slots 1A and 1B. Now, player 1B has the card as well. You can repeat this as many times as needed. Note: The Red Dragon is transformed from a Lizardman or a Red Lizard after getting 6000 experience points.
When you complete Blessoon, you can go talk to Gurd at the Apothecary. She will tell you of a place with giant bees. You must eliminate them in Dahl-Nok Valley to complete the level.