i'd say 25man raid COT:northend
we are helping the arthas and he's human army to hunt down mal'ganis and claiming the frostmourne make it long raid but not that long with tons of endless adds but lots of bosses:
it start by landing in northend
we help arthas to destroy the ships
and we meet the dwarfs
then there's a fight where arthas ask us to defend the base while he go find the frostmourne with the dwarfs then he come back alone ofc :P
and it end up 25ppl arthas and the human fighting mal'ganis in hes base and hes army lots of undead adds to aid him and you need like 4-5 tanks to tank adds and
kiting and brains or whatever somthing like vashj fight ;p