How do the new Dark Eldar interact with the Kill Team rules in the Battle Missons rulebook?
If there is nothign in the Kill Team rules that says individual models are considered to be a single squad of 1, then I would say the pain tokens are given to the entire squad (a squad being one elite, troop or fast attack choice) for wiping out an entire enemy squad.

However, if the Kill Team rules explicitly state that each model is conisdered to be its own squad, then I would say pain tokens are given on an individual model basis for each model they wipe out.