Look, It will be hard, but have faith get a very, very, good sword with a strong water attribute and if some good friends with good water weapons, send two on each Diablos and when one team has finished, send them to help the other team
I sugest that everybody has a frostedge+, and strong armor with the Autotracker ability
(Ioprey Mask
Yian Guragu Mail,
Guragu Vambraces,
Guragu Tasset,
and Basarios Greaves or Hunters Greaves.)