Pikachu isn't very strong so... (I think this has what it takes for competitive)
Pikachu @ Light Ball
Ability: Lightning Rod
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Volt Tackle
- Iron Tail
- Volt Switch
- Brick Break / Nuzzle
This set is rather risky, but with Pikachu's low stats and a high chance of dying in one hit, one has to make use of its limited turns. Because Pikachu is going down with one hit, Volt Tackle is a great option to do as much damage as possible. Pikachu is also decently fast, and with the Jolly nature is almost guaranteed to go first. Volt Switch is good if you predict the opponent switching, and it will even save your Pikachu to come back in later. Iron Tail is good for heavy damage with no recoil, however has low accuracy so I use it as coverage. Same goes with Brick Break (which also has the ability to break through light screen and others). If you don't want or need coverage, Nuzzle or some other status move can be used to paralyze them.
Pikachu isn't really for competitive but its one of my favorite Pokemon.