You can locate the Banished mobs in the Northern part of Dereth or the Southern part from portal Land Bridge Villa Portal at 74.9S 17.9E (Qalaba'r). I find them a lot from the Land Bridge Villa drop to around 10W. There's no timer on picking up the weapons and every kill drops one. Good luck!

Banished Grievver	->
Banished Lugian	->
Banished Banderling	->
Banished Shadow	->
Banished Phyntos Wasp	->
Banished Olthoi	->
Banished Monouga	->
Banished Mu-miyah	->
Banished Tusker	->
Banished Tumerok	->
Banished Drudge	->