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Here's the final gym for my American region. Gym is just a haunted house with trick/trap doors forming a labyrinth and trainers hiding (like in suits of armor or behind shelves or curtains) to scare you.
Gym leader uses trick room, high defenses, recovery, and the core elements for offense and resistance as a way to highlight the lessons a player should learn through the game. Only 3 items, but the leader will switch often to take advantage of abilities (flash fire, water absorb, mummy). I know jellicent isn't primary ghost but there are no primary ghost with water secondary types, so its a shoe in to make it work with the grass, fire, water core typing.
But the leader is designed to be very formidable, and honestly designed to really push the player to form a strategy against them other than hyper offense. Ideally the leader should get a 25% win rate against first challengers.