Instructions
Catch Psychic-type such as Kadabra, which can be found wild on Route 8. You can also get a Kadabra by raising an Abra to Level 16. Kadabra is a powerful Psychic-type Pokemon with a good speed stat, making it useful for the upcoming battles.
Make sure the other Pokemon on your team have special attacks. Normal- and Fighting-type moves are ineffective against Ghost type Pokemon and will be unable to harm them. Psychic, Ghost, Dark and Ground type moves will all be super effective against Morty's team.
Level up your team in advance by battling wild Pokemon. Morty's Pokemon range in level from 21 to 25, so your team should be around the same or higher to ensure an easy victory.
Stock up on potions at the Ecruteak City Poke Mart before challenging the Ecruteak City Gym. Buy Parlyz Heal and Revives, as well, to keep your Pokemon alive through the battles ahead.
Head into the Ecruteak City Gym. Battle the various trainers along the way. Since Ecruteak trainers all use Ghost-type Pokemon, use your Psychic-type to counter them, bringing in other Pokemon as necessary. Getting through the Gym can be tricky due to the invisible floors, as you can't simply walk straight ahead. After beating the first trainer, head right a few steps, then turn and head upwards until you face the next trainer. After you beat her, head left, then up again a few steps later to face the third trainer.
Save before challenging Morty.
Press "A" to talk to Morty and initiate the battle for the fourth badge.
Attack quickly with special attacks, healing your Pokemon with Potions if necessary. If one of Morty's Pokemon paralyzes yours with the move Lick, use a Parlyz Heal to recover from the status ailment. Once you beat him, Morty will give you the Fog Badge, which allows you to control Pokemon up to Level 50. You will also receive TM 30, which teaches one of your Pokemon the move Shadow Ball, a powerful Ghost-type move.