Forget Nergigante and Kushala Daora. Vaal Hazak is giving me the most trouble lately. Between its wacky waving inflatable arm-flailing tubeman head and tail movements, the miasma smokescreen, and the fact that it flails and charges almost as much as Odoggo, breaking its parts is a huge pain. With a full group of hunters (provided CRAPCOM's trash netcode can serve up even 1-2 available SOS), it's much faster and easier, but it's a die roll then whether or not parts will get broken (they probably won't).
I've come up with a solution. The Karma LBG piece of shit has finally come in handy this one time. When Vaal Hazak does its fart breath, you can't melee its head effectively because it creeps forward as it does the attack; whenever its forelimb move forward, you get tapped out of any animation you're in, particularly combos. Ranged weapons suffer from no such issues. Probably the bow would be more effective at this too, but I figured I needed to justify the effort I put into making Karma somehow.
DragoFireheart I've love to see a video of you soloing this fartfuck with a LBG. As soon as its face breaks, I farcaster back to camp to switch over to a non-trash weapon.
On a slightly different tack, I'd say the difficulty of most fights depends strongly on the number of players. It's about like this, listed from easiest to most difficult:
three or four competent hunters
solo
three or four mixed-competence hunters
competent duo
three or four incompetent hunters
incompetent solo
incompetent duo
I've done a lot of fights with 3-4 people now, and when we're all competent it's easy as fuck. Duoing is overall the hardest (certainly most time-consuming) way to play, hands down.