Pikachu is a shining example of why Power isn't everything. Doesn't matter if you hit comparably to Groudon when you have the defenses of wet rice paper. Seriously, Dugtrio compares favorably to Pikachu defensively and Dugtrio's defensive utility is non existent. Base 90 speed is not bad, but on something as frail as Pikachu, it is far from sufficient.
Ability: Lightning Rod
Item: Light Ball
EVs and Nature:
4 HP / 252 Atk / 252 Spe
Physical Pikachu is an even worse idea than Special Pikachu. Volt Tackle is the main reason why. Yes it is Electric Double Edge... And Pikachu has base 35 HP, one of the lowest in the game. Unless you are Volt Tackling Wugtrio, the resultant Recoil Damage is going to maim if not outright OHKO Pikachu. But hey it hits hard. Fake Out is Priority and easy chip damage, helping Pikachu reach KOs. Play Rough is coverage against Dragon types, a noticeable weakness on Special sets. Volt Switch enables Pikachu to pivot out to an actually effective teammate and is also STAB that wouldn't kill it.
Lightning Rod is recommended for ability. As without it, Pikachu is worse than Wugtrio for defenses (yes an immunity means so much). Light Ball is mandatory for Pikachu to be worth considering, so if Pikachu is Knocked Off (and is miraculously not KO'd), or Tricked, it is out of the fight.
Also some fun damage calcs:
252+ Atk Light Ball Pikachu Volt Tackle vs. 0 HP / 0 Def Iron Bundle: 446-528 (176.2 - 208.6%) -- guaranteed OHKO
252+ Atk Light Ball Pikachu Volt Tackle vs. 252 HP / 252+ Def Dondozo: 326-386 (64.6 - 76.5%) -- guaranteed 2HKO
252+ Atk Light Ball Pikachu Volt Tackle vs. 248 HP / 252+ Def Corviknight: 350-414 (87.7 - 103.7%) -- 18.8% chance to OHKO
252+ Atk Light Ball Pikachu Volt Tackle vs. 248 HP / 252+ Def Pelipper: 720-852 (222.9 - 263.7%) -- guaranteed OHKO
252+ Atk Light Ball Pikachu Volt Tackle vs. 4 HP / 0 Def Wugtrio: 866-1022 (408.4 - 482%) -- guaranteed OHKO