Normal Behemoth isn't that hard, although it's still heavily RNG. You can however farm the set without actually killing it, using Palico's Plunderblade and breaking the parts(tail cut with a partbreaker slicing ammo LBG, claw/horns with sleep > bombs and sticky). Requires an hour or two of farming, especially claws are shit to break but you can get all your shit done and then look into actually killing it properly. Can try looking for a session that has Behemoth too where people might be less retarded about it.
And for Behemoth, and hard shit in general, 2-3health boost is great when you're not a super pro, eating for defensive food also works well enough since damage is rarely an issue in parties, and you want to augment your armor and upgrade it to the max if possible(I mean upgrade to the max for sure, but augment+upgrade to the real max if you have the mats for that). For behemoth specifically there's also a few things, you want to bring astera jerky to remove bleed, flashes to interupt his shit(as well as flashbugs to make more flashes), instant heals like max potions, defensive mantles and shit.
Not too sure about Gunlances iirc most of the good ones are from KT and ATKT.