When looking at Pikachu's stats you might wonder why anyone would use this over Plusle or Minun. The answer is it's signature item, Light Ball, which doubles it's special attack making it the most powerful special attacker in the tier excluding the occasional Clamperl and with few electric and ground types, Pikachu can hit most of the tier for at least neutral damage and it also has access to surf which can help dealing with ground, rock and fire types. However, it's defenses are pathetic making Pikachu very easy to revenge kill by faster pokemon or pokemon with priority moves.
move 1: Thunderbolt
move 2: Surf / Hidden Power Ice
move 3: Substitute
move 4: Encore / Hidden Power Ice
item: Light Ball
nature: Hasty / Mild
evs: 252 SpA / 4 Atk / 252 Spe
Thunderbolt is Pikachu's main STAB move to deal with water types like Huntail, Wailord and Kingler and flying types like Pidgeot, Murkrow and Golbat. Surf can deal with rock, ground and fire types such as Sudowoodo, Pupitar and Flareon. Hidden Power Ice gives Pikachu perfect coverage allowing it to hit the entire tier for neutral damage and also helps it against grass types like Roselia and Bellossom. Substitute allows Pikachu to attack one more turn before fainting to almost any attack. With a bit of prediction Encore can be very useful. Since the opponent will be locked onto one move, it allows Pikachu to KO some pokemon that thought they could set up on it.
Since Pikachu can't take any hit, it's defenses are pretty much there just to be lowered. Hasty allows it to outspeed more pokemon like Pidgeot and Plusle while also outspeeding pokemon that would normally cause a speed tie like Furret and Vigoroth.
Pikachu can really switch only after one of it's allies fainted or if it's the lead, basically Pikachu just needs a way to use substitute so then it can KO top threats like Chimeco and Mawile safely.
If it doesn't have Hidden Power Ice then Pikachu appreciates some help from pokemon such as Flareon and Pidgeot since they can hit both Roselia and Bellossom with their super effective STAB moves. Pikachu also appreciates help from grass types like Bellossom and Roselia to help it against Whiscash. Since Pikachu will die to pretty much any priority move such as quick attack from Pidgeot and mach punch from Hitmonchan, Haunter and Sableye can switch into both since the possess Normal and Fighting-type immunities. Arbok can switch into Hitmonchan pretty safely thanks to it's resistance to fighting type moves and access to intimidate.
Thunder Wave can be very helpful if you are running a Mild nature instead of a Hasty one , Volt Tackle is an option if you want more power but it's a bit too risky if used with Substitute. Pikachu can also use Hidden Power Grass for Whiscash.
Checks and Counters
Whiscash is a threat to any Pikachu that is not carrying hidden power grass due to it's immunity to Thunderbolt and it only taking neutral damage from Surf. Priority users like Hitmonchan and Pidgeot beat Pikachu.