A little experimenting today.. Went to bridge after start of 14.. Stayed all of 14, using Retriever only.. Stay MOST of 15, Retriever only.. Went back to Alcatraz, pit wouldn't not keep my Ax.. Went back to bridge end of 16, stayed ALL of 17, left very beginning of 18, only used Ax.. Got back and got my Redeemer.. My theory, stay one COMPLETE round, using only the Retriever, and you can get the Redeemer.. And multiple players can do it at the same time it has been confirmed, but whether or not it takes a certain amount of kills, I am unsure.