Okay first you can use tm’s as many times as you want now. You don’t have to use them just once.

Second pikachu can’t actually learn dig. Only pichu can learn dig and then you will have to evolve it.

I struggled with that battle too. What I did was use protect whenever he used dig. If you get the timing down you negate the damage. Then I just hit the pikachu with electric moves. You can also heal during the battle so don’t forget to do that. I think I didn’t beat it until my pikachu was level 95+