A Gligar is a Flyscorpion Pokemon that first appeared in Generation II. It is entry #207 in the Pokedex. A Gligar cannot evolve into a Gliscor without the special held item, Razor Fang. Evolving a Gligar requires a conscious Gligar, a Razor Fang and correct timing of leveling up.
Obtain a Razor Fang
In Pokemon Diamond, Pearl and Platinum, you can find a Razor Fang on Route 225. In Pokemon Black or Pokemon White, you can find a Razor Fang on Route 13 or at the Abundant Shrine. In Black 2 and White 2, Razor Fangs are found along Route 11. Depending on your version of Pokemon, you may be able to buy a Razor Fang at Battle Frontier, Battle Park, Battle Maison or Gear Station.
Equip Gligar with Razor Fang
A Gligar must hold the Razor Fang to stand a chance of evolving into a Gliscor.
Wait until in-game nightfall
Gligar can only evolve at night while holding the Razor Fang. Equipping a Gligar with a Razor Fang during the day gives the item's regular bonus, but does not affect its evolution.
Level up Gligar
Select Gligar for battle and ensure it wins XP to move to the next level. If a Gligar receives enough XP at night while holding a Razor Fang, it both levels up and evolves into a Gliscor.