Ok here's some advice. Kushala Doara is one of a select few monsters to actually be considered as 'dragons' rather than 'wyverns' (wyverns have 2 legs rather than dragons, which generally have 4), he is classed as an 'elder dragon' to be exact.
The 'elder dragon' class of monsters consist of Kushala Doara, Lunastra, Teostra, Fatalis (and its variants), Chameleos, Kirin, Yama Tsukami, and Lao Shan Lung; all of which can be repelled rather than killed, except the Kirin and Yama Tsukami which must both be killed in a single quest, (the giant carapaceon (crab monster) known as Shen Gaoren can also be repelled but is considered a carapaceon rather than an elder dragon).
Discounting the Kirin and Yama which both must be killed in the one hunt and cannot be repelled, the elder dragons; Kushala Doara, Lunastra, Teostra, Chameleos, and all breeds of Fatalis may all be fought over multiple quests. To explain further, elder dragons are very bold and majestic creatures which will often return to their habitats EVEN AFTER YOU'VE ALREADY REPELLED THEM! And, since it's technically the same monster you're fighting from a previous hunt, the damage you dealt to them will still be there the next time they are sighted and you decide to hunt them again. This means that the health you took from them will still be gone (unless they were very near-death when you repelled the dragon previous) and if you broke any of their horns/cut off their tails etc. - they will still be gone in the next hunt!
The thing is though to fight an elder dragon you've repelled previously, it must be in the same difficulty and rank quest level as the last time you hunted it.
Now, each gigantic elder dragon aka Lao Shan Lung and each gigantic carapaceon aka Shen Gaoren along with the giant floating octopus, Yama Tsukami, can only be hunted in a single quest, so you won't face that individual monster again once the hunt is over, regardless of its outcome (although you can hunt a different one of those two monsters at any time, but your damage caused from the last time you fought them will not carry over as it is not the exact same monster, if you can understand me).
Additionally, because elder dragons (EDs from now on) are so rarely-sighted by the villagers, their hunting quests will not be available all the time. To make them reappear, you need to go do some different hunting/slaying quests in order to get the opportunity to hunt them again.
Ok back to the Kushala ED. As MasterOnion said Kushala may return to a different habitat, but it is still the same Kush, as long as he's in the same level of quest given by the same quest dealer. You'll be pleased to know that Kush's main weakness is indeed Thunder (all EDs also have some weakness to the 'dragon' element, some more than others). You'll also be pleased to know that by either poisoning the Kush or cutting of his horns/tail you can cancel out his wind aura ability allowing you to attack him freely without worry of being thrown back by his wind press aura.
I'd say you'll kill that Kush in the next (third) quest. It's usually only Fatalis and its variants (Crimson Fatalis and White Fatalis) which usually take 4 quests to kill.
Hope I helped, if you need any more info, please google the Monster Hunter Wiki.