252- SpA Light Ball Pikachu Thunderbolt vs. 0 HP / 4 SpD Mewtwo: 160-189 (45.3 - 53.5%) -- 34.8% chance to 2HKO
252 SpA Life Orb Raichu Thunderbolt vs. 0 HP / 4 SpD Mewtwo: 164-192 (46.4 - 54.3%) -- 60.5% chance to 2HKO
Life orb Raichu hits harder, is much faster, and has better defenses. There is no reason to use Pikachu.