I've been sequentially ticking off things to complete the collections for the griffon from north to south and have run into a wall in the Elon Riverlands.
The deadhouse event seems to at always have someone waiting for it to start but the Riverlands are so dead that advertising the event in map chat never yields more than 6 people to attend. With out a doubt, a group of 6 will wipe and fail the fight with Jabari. In talking to others who need the achievement they too have been trying for multiple days with no success.
Last night through proactively switching maps to find people from busier areas who could help, I managed to get 10 people to try, we failed because once you get Jabari down to about 25% adds get spammed very hard. After the 18 minute cooldown passed I managed to get 12 people to try, making sure to destroy the things that spawn adds before fighting him, but there was not enough time to defeat him once that was done.
The amount of effort it took to organize enough people to attempt it is unrealistically high given how dead the map is. It seems you need more like 20 people to succeed... And in a low population area that's not feasible anymore...