I know this thread is old but I happened upon it and thought I would share my knowledge for any other interested parties.
I use the following build to farm tengu in HM in North Kryta Province, Though a run generally takes me about an hour but that's because I farm the fire imps and scale as well. In HM between actual feather drops and scalps I can average one stack of feathers in about 2 rounds.
Please note this build was not orginally mine I can't recall where I found it but I know it's used for Destroyer core farming as well.
E/ME

Code:OgVEMIWJz5QLMAb1SxXV6rN4hA
Inspiration Magic = 8
Air Magic = 5
Earth Magic = 12 +4 (superior earth magic rune)
Energy Storage = 9 +1 (Minor Energy Storage Rune)
Full Geomancer insignias
Staff:
Earth Staff
Adept Staff head
Enchantments last 20% Longer Chance 20%
The trick to to make sure you precast 5,6,7,& 8 before aggroing the tengu so as to make sure that the mesmers can't frustrate or degen you. Keep a constant cast while they are pounding on you as the brave's can cause bleeding but you can survive through it by keeping up AOR(skill 1) and constantly casting your other skills. DON'T FORGET TO MAINTAIN 5,7,& 8 else you will die. Move in close to all enemy, the rangers can take a bit when there is only one or two but you can help it along by wanding them.
I hope this information helps other ele's out there