So in short, it can be any kills of the opposite faction(or same faction if in ffa)
So, the process can be sped up by having level 1 ressing at the spirit while your character with the legendary kills him.
Bring level 1 to any neutral graveyard.(opposite faction)Bring your character with legendary to the same graveyard. Kill level 1Resurrect at spirit.Repeat until you get 1000 kills. Profit!