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Ace would be Mega Steelix or Flygon depending on whether megas would be acceptable or not
The gym would be in some open-top sandcastle to take advantage of the town's natural sandstorm. As you make your way through quicksand puzzles, you feel the ground rumbling below you at various points. When you reach the gym leader, Steelix/Onix emerges from the ground next to him. Probably leads with Trapinch or Sandile.
TM would be Bulldoze or Earthquake, depending on where you are in the game.