Shonisaurus is the only fish I can think of in Witchdrop that would require a buff to catch. I see it's still unlisted as to where in the OP post but it should be pretty obvious to be in Witchdrop just no info on how. As ServerCollaps said in a previous post it's most like catching a certain number of Rhamphorhynchus which means it's going to be a nightmare. Or it's catch a certain amount of Mahar's which is less of an RNG nightmare in terms of fishing as you just need to wait for weather RNG instead (Fair/Clear->Blizzard).
No idea on if it needs mooch or a different bait but best bet is always to gamble on Cloud Cutter mooch I guess, especially as it just looks like a spiced up Rhamphorhynchus/Mahar.