I had a lot of trouble trying to get this completed. I did this while using Thor as he was my highest level character, but this will work with any character. This was the easiest way I found how to do it. Open up the mission 'Prototype Weapons Forge' in the Utah Badlands (This mission might not appear for you as it is a Warzone and they change on the war table daily). Select 'Challenge I' as it will be the easiest difficulty of enemies you will encounter. Once in just sit back and let your companions do all the work. If you have any issues I've attached my video below so you can get an idea of how I did it.