One character must be a Siren.
Go back to Fyrestone in the Arid Badlands; and after you kill the Mothrakk a Eridian weapon will fall out but you have to search everywhere as the trash plot can be huge. When the Mothrakk is low on points shoot warning shots to get it to come towards you, then kill it on the approach.
Berserker: Eridian Cannon
Soldier: Thunder Storm
Siren: Ball Blaster or Wave Blaster
