"The pincers on both of its arms become even bulkier, featuring sharp barbs that can stab right through concrete. Any opponent that gets caught by Mega Scizor's pincers will have no hope of getting away."
Scizor was caught by Tierno during his journey through Kalos. It is unknown whether it was caught as a Scizor or if it evolved in Tierno's care.
Lexi received Scizor in The Calm After the Storm as a gift from Shauna who got it as a gift from Tierno's parents. Shauna decided to give Scizor to Lexi, as Scizor is a keen battler, something which Shauna wasn't. Lexi happily took him in her care. She used him during her battle against Drasna of the KalosElite Four in which it lost. After the battle, he was sent to Hayley who now lived in Kalos and opened a new Ranch. Lexi however promised him that she would bring him back to her team soon.
Ruby & Sapphire
Scizor was brought back onto Lexi's team in Hoenn after she decided she needed to have strong Pokémon with her and also so she could utilize their ability to Mega Evolve. Scizor initially had a small struggle with its new strength, but because of his prior bond with Tierno, Steven explained it was possible that Scizor carried that bond with him to help him as Mega Scizor.
Scizor was kept in reserve following this, training at Lexi's Secret Base until he was needed.
Scizor has steel pincers, which can crush any hard object he gets a hold of into bits.