Well here I go....
Scizor was one (If not the most threatening) Pokémon in last generation. The coming of new Pekémon who also compete for his roll, such as Excadrill with its high Attack stat and blinding Speed in Sand (That is so common this gen), and Conkeldurr with its humongus HP and Attack backed by Priority in Mach Punch, has somewhat disminished is usefulness. But do not let this discourage you from using Scizor. With its excellent typing, and useful resistances, impressive Attack, and the right moves, Scizor can fit into any team, and be a menace for the teams who are not prepared for it.
Swarm: Not a really good ability, but useful anywhow boosts your Bug Type moves by 1.5 when HP is below 1/3 of health.
Technician: A really good ability and one of the main reasons why Scizor is a beast. It powers up moves with 60 Base Power or less by 1.5.
Light Metal: Halves you weight, yeah not useful.
Big Bad Bug
Item: Leftovers/ Life Orb
EV's: 248 HP/ 212 Sp. Def/ 48 Atk
Bug Bite/ Pursuit/ Superpower
This set takes advantage of Scizor's array of resistances. With the HP and Sp. Def investments it can set up with ease on resisted special attacks, such as Latios/ Latias/ Hydregon Draco Meteor (When they are Choiced), Reuniclus Psychic, Politoed Ice Beam, Gengar Shadow Ball, etc. Switch into any of this, Swords Dance as they switch, to skyrocket your Attack, and proceed to destroy stuff with a Priority Bullet Punch, and Bug Bite, when you're low on health Roost. Rinse and repeat.
The other moves have each their own nich. Pursuit when you have already Swords Danced will blow the candle of Chendelure who would otherwise destroy you with a fire move. Superpower is useful when you can get Heatran on the switch.
Item: Choice Band
EV's: 248 HP/ 252 Atk/ 8 Def
U-Turn/ Bug Bite
Superpower/ Brick Break
Pursuit/ Quick Attack
Back from 4th Gen the powerful Choice Band set returns. Bullet Punch is your classic Priority move denting anything that doesn't resist it (And even some things that do). U-Turn is the move for scouting your opponents team while dealing a lot of damage. Bug Bite gets a Technitian boost. Superpower and Brick Break are your Steel killer moves who would otherwise wall you. Pursuit is for the fleeing Pokémon that think they can escape your rage. Quick Attack gets a mention since its your only way of hitting Thundurus and Zapdos.
Fire Types are your best bets in here, being Scizor's only weakness. And a few ones come to mind, mainly Heatran and Chandelure. Both resist Scizor STABs and cand fry it with a Flamethrower or a Fire Blast. Steel types also get a mention specially Skarmory and Ferrothorn but none can do any notable damage to Scizor, but they can get up Spikes. Be careful when using Skarmory because if it Roost's while Scizors Superpower's you get OKHO'ed.