Let's say the Loki/Chitauri scenario happens in the Buffyverse. A composite Buffyverse where all members of the Scoobies and Fang Gang, and those who have fought alongside them, are alive at the same time (TV series only). Those members are:
Daniel "Oz" Osbourne
Allen Francis Doyle
Principal Robin Wood
They're forced to join forces in New York as the only legitimate defence against the invasion.
With the same kind of build up as the Avengers movie (so they know what's going to happen), can they prepare for the invasion and stop it? They can call in any help they would have available to them (for example the Initiative, Devon Coven, Wolfram and Hart, Gunn's gang etc - assuming they would all be willing to help).
Notes: Spike and Angel are both ensouled, they both have Gems of Amara to negate their vampire weaknesses (most importantly to sunlight), Buffy has her slayer scythe, Illyria still has her time powers (as displayed on TV only) and other team members have weapons of their own choosing.
Have they got enough going for them to stop the invasion and defeat Loki?