We have some good news for you! Although it’s pretty difficult locating Diglett in the wide open areas, it’s much easier in the cramped confines of a cave. Plus the terrain is simpler (with less vegetation etc.), so the Diglett stick out like sore thumbs.
1/7: Starting from the Soothing Wetlands entrance, turn right and follow the river (but don’t enter it yet). Head for the cavern wall directly ahead. It’s behind the boulder towards the left.
2/7: Swim across the river, or come here via the Training Lowlands. Go up the nearby slope. When the ground levels off and there’s a rocky pillar towards the left, check behind the boulder to the left of the next slope.
3/7: From Diglett 2, go up the second slope. When the ground levels off again, check behind the boulder that’s immediately on the right.
4/7: From Diglett 3, face the nearby Pokémon Den. Then head in that direction. Soon, you should catch a glimpse of it to the right of a boulder.
5/7: From Diglett 3 and 4, turn left from the second slope. Stop after you go past a rocky pillar and before the next slope. Look to the right. Can you see it to the left of a boulder?
6/7: From Diglett 5, go up the third slope. When the ground levels off, turn right. It’s located in front of some rocks, to the right of an item ball.
7/7: From Diglett 6, continue towards the exit, which leads to Challenge Road. Just before the exit, it’s on the right. Or you can come here from Challenge Road and it’ll be on your left.