There are several episodes in which Pikachu is able to hurt ground-type Pokémon (such as Brock's Onix) with electrical attacks. Electric-type attacks cannot harm a ground-type Pokémon, no matter how much stronger the electric attack is.
In this episode, Ash's Pikachu beats Brock's Onix with his electric attacks. Since Onix is both rock and ground type, it would be immune to all electric attacks.