I've tested on "An Impossible Foe" - but it may work for others too.
Buffs such as:
- Sylvan Elixir - 10% Bonus to stats
- Broken Isle Food - food such as Spiced Wildfowl Omelet - 10% increased haste after killing an enemy (and there's plenty in this one)
- Class Hall Shaman Buff (if you picked the trait) -- try other class hall buffs too.
All work. You have to use them INSIDE, not beforehand. Using these will allow you to do them at a much lower item level. I finished this on an 889 alt just using those two buffs.
Others do too possibly, but I don't know them off the top of my head. Perhaps some people wanna go try some more and find out?