How to Get Shadow Shuckle in Pokemon Colosseum

Play through the game and beat it to finish the main storyline. Go to Pyrite Town. Find the secret passage by a bookshelf in the town. Battle Cail, and then talk to him to get the map to the Team Snagem Hideout. Head over to the Team Snagem Hideout and battle Snagem Head Gonzap. After you beat him, he will drop the D-Disk on the table. Pick up the D-Disk.

Head to "The Under" of the Colosseum. Search for the giant hidden automatic door that will open when you are carrying the D-Disk. Enter the door and go into the Deep Colosseum.

Enter and beat the Deep Colosseum challenge battles four times. Enter the challenge once more to fight Shadow Shuckle's trainer, Deep King Agnol, in the final match. Battle his Pokemon. When Shadow Shuckle is sent out during the battle, weaken it by making its health bar go into the red, and then freeze, paralyze or make it fall asleep with one of your Pokemon. During your next turn, with your first Pokemon, go to the "bag" menu and select the pokeball you want to use to catch the Shadow Shuckle. When your second Pokemon's turn is up, go to the bag, and press "X or "Y" on the selected pokeball. Move the cursor to another pokeball. Press "A" to swap the pokeballs.

Battle with your second Pokemon if Deep King Agnol still has more Pokemon to use in the fight. Your character will throw the pokeball you selected at Shadow Shuckle. Press and hold the down arrow and press "A" repeatedly as fast as you can, and you will catch the Shadow Shuckle.