You get it from an ancient shard (5* volcano area online only)
You can also obtain Ancient Shards from a couple of other High rank quests.
um dont listen to what anyone else says, to get the gun is so easy you just play offline mode till you get the trade with argosy which is a 3* RARE ITEM TRADE you get the gun as simple as that
I'm sorry, but to the 3rd comment, thats for Aquamatic "Needler". Not Longshot.
To get the Longshot, just keep grinding out Ancient Shards til you get one. Its all there is to it.