for the black gravios, definately use a bowgun and shoot water shots trough his stomach. it makes him 10x easier and its quick too.
i would recommend the HR lobster LBG, rapid firing water shots at his belly is extremely effective, since it is is raw and element weakspot once you break of the belly plate. and when you're out of water shots, fire normal 2 at his belly (it should have broken of by that time) and he'll be dead in no time.
for armor, use either the kirin armor for element attack up or obituary/butterfly armor for evade +1/2.
for black diablos, i would recommend firing pellet shots at it's tail, but i don't remember any good pellet sets/bowguns at high rank anymore.