This is a fairly small zone, with a small number of Diglett. In theory, you shouldn’t have too many problems, but a needle in a smaller haystack is still a needle in a haystack!
1/8: Starting from the Training Lowlands, go up the first flight of stairs. It’s sitting comfortably next to the vegetation past the boulder on the left.
2/8: From the lowlands entrance, go up both flights of stairs. When you reach the crossroad, go straight ahead. Walk around to the back of the tall, yellow grass. It’s by the cliff edge, facing the lowlands entrance.
3/8: Starting from the Brawlers’ Cave entrance, turn right. It’s buried next to a small rock by the cliff wall, near the cave entrance.
4/8: From Diglett 3, travel forward with the cliff wall on your right. Where the cliff wall ends, it’s hiding next to some vegetation that’s overlooking the ocean.
5/8: Head for the central crossroad, where the first oriental lantern is. Go up the slope to the next lantern. Number 5 is hiding near the boulder and vegetation to the right of the second lantern.
6/8: From the second lantern, while facing the Tower of Darkness (at the top of the stairs), turn left. It’s hiding at the edge of the path, overlooking the Training Lowlands.
7/8: From the second lantern, while facing the tower, turn right. Go up the slope that leads to a Pokémon Den. It’s found right behind the den.
8/8: Go up to the Tower of Darkness, at the highest point of this zone. After climbing up the stairs, it’s immediately on the left, near some shrubbery.
By our count, that’s exactly 70 Diglett returned to their owner. You’re almost halfway there, but not quite. That also means there’s still no reward. Next time, we promise!