You can get a Gligar with it's Hidden Ability Immunity in the Hidden Grotto on Route 23.
Here is a picture of the Hidden Grotto on Route 23:
Note Gligar is the rarest of all Pokemon that can be found is this Hidden Grotto as there is only a 1% chance of encountering a Gligar.
To get a Gligar with Immunity with a Jolly Nature you're going to have to be either extremely lucky and get the first Gligar you capture from the Hidden Grotto with a Jolly Nature, or breed the Gligar you captured in the Hidden Grotto until you get one with Immunity and a Jolly Nature.
Of course if you're breeding the Gligar you've captured it has to be a Female, as Males can't pass down their Hidden Ability in Gen V.
To get a Gligar with Immunity, a Jolly Nature, Night Slash and Cross Poison you're going to have to breed your Gligar with a Drapion, (you can breed it with other Pokémon but Drapion is the preferred choice as it can learn both Night Slash and Cross Poison by levelling up.)
Once you've bred Gligar enough times hopefully you should have a Gligar with Immunity, a Jolly Nature, Night Slash and Cross Poison, all that's left to do now is evolve Gligar into a Gliscor with a Razor Fang.
And of course you can teach Gliscor Earthquake and Swords Dance through TM26 and TM75.