Mega Pinsir is a very straightforward Pokemon:
-Swords Dance
-Return
-Close Combat/Earthquake
-Quick Attack
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Pinsirite
Ability: Hyper Cutter/Mold Breaker
Mega Pinsir's biggest niche is in Aerilate Quick Attacks. While it's not as threatening as Talonflame in priority Flying moves, this is still useful for revenge killing purposes. Pinsir also pairs well with other Flying Pokemon, mainly Talonflame and Staraptor, for wearing down each other's checks and counters:
Talonflame:
-Swords Dance
-Flare Blitz
-Brave Bird/Acrobatics
-Roost
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Sharp Beak/Sky Plate/none
Ability: Gale Wings
or
-Swords Dance
-Brave Bird/Acrobatics
-Taunt/Will-o-Wisp
-Roost
Nature: Adamant
EVs: 248 HP/252 Atk/8 Spe
Item: Leftovers/none
Ability: Gale Wings
or
-Flare Blitz
-Brave Bird
-U-turn
-Steel Wing/Tailwind
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Band
Ability: Gale Wings
Staraptor:
-Double-Edge
-Brave Bird
-U-turn
-Close Combat/Final Gambit/Quick Attack
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf/Choice Band
Ability: Reckless
As all three of the above Pokemon are weak to Stealth Rock, Starmie makes a nice Rapid Spin user to clear off said hazard:
-Hydro Pump
-Psyshock/Thunderbolt
-Ice Beam/Hidden Power (Fire)
-Rapid Spin
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Ability: Natural Cure/Analytic
or
-Scald
-Psyshock/Reflect Type
-Rapid Spin
-Recover
Nature: Timid
EVs: 252 HP/4 SAtk/252 Spe
Item: Leftovers
Ability: Natural Cure
Therian Landorus is a good teammate for the above to get each member in safely with a slow U-turn and set up Stealth Rock:
-Stealth Rock
-Earthquake
-Stone Edge/Toxic
-U-turn/Swords Dance
Nature: Impish
EVs: 248 HP/244 Def/8 SDef/8 Spe
Item: Leftovers/Rocky Helmet