Shadow Pokemon
Shadow Pokemon are the main type of Pokemon you use for most of "Pokemon XD." In most cases, Shadow Pokemon outclass their normal counterparts. Shadow Pokemon possess Shadow moves that can be used an unlimited number of times and are always super-effective against normal Pokemon. This allows these Pokemon to devastate any normal Pokemon. In addition, Shadow Pokemon resist Shadow moves, which negates the main damage ability of almost every enemy Shadow Pokemon.
When to Purify
The ultimate goal of "Pokemon XD" is to purify the Shadow Pokemon to turn them back to normal. Given the overpowering nature of Shadow moves, this seems like a bad idea. However, Shadow Pokemon cannot increase in level, which limits how long they can be useful. The benefits of being a Shadow Pokemon become less significant once enemies have a level advantage. Purify Shadow Pokemon once you start facing enemies higher in level than them.
Master Ball
The Master Ball is an item found in most "Pokemon" games that allows the player to instantly capture a Pokemon without worrying about weakening them first. There is only one per game, which makes its usage pretty significant. The best strategy is to use this on the most powerful Pokemon in a game, eliminating the most difficult battle. In the case of "Pokemon XD," it should be used on Shadow Lugia, which Grand Master Weevil summons near the end.
Easily Beat Grand Master Weevil
Grand Master Weevil is the final boss in "Pokemon XD." Even though you captured his most powerful Pokemon, Shadow Lugia, before the final battle, he still has a strong team made entirely of Shadow Pokemon. You are also required to capture all of his Shadow Pokemon during the fight, making it even more difficult to complete normally. There is actually a very cheap, easy method to win this fight if you stock up on healing items and Timer Balls beforehand. Use Shadow Lugia and stall the fight for about 40 turns by using healing items to keep Lugia at full health. Grand Master Weevil's Pokemon have only Shadow moves, which do little damage against Shadow Lugia. After 40 turns, use your own Shadow moves to weaken his current Pokemon to about 50 percent health. Throw Timer Balls at it until you capture it; because of the 40-turn delay, the effectiveness of the Timer Balls has increased significantly. Repeat this to capture all of his Pokemon easily.