...pick one of two options. Either try to find Clay and use his Excadrill to quickly dig a tunnel leading straight to the Gym (but you don't know where he is or if he's even here), or use the TM28 you have to teach one of your Pokemon Dig (preferably Elizabeth, since she can evolve into Nidoqueen, so Dig won't be as much of a waste on her), but her digging will take much longer than Clay's Excadrill. You ask Serena which she thinks is faster, and she says...