Cut Your Losses by paying ({4} magic symbol) ({U} magic symbol) ({U} magic symbol) , causing a player to mill half their library, rounded up.
Activate
Riverchurn Monument's exhaust ability by paying ({2} magic symbol) ({U} magic symbol) ({U} magic symbol) and tapping it, causing each player to mill cards equal to the number of cards in their graveyard, thus causing the player targeted by
Cut Your Losses to mill the rest of their library.
Notes
You may use
Cut Your Losses's casualty ability, if desired, to cause two players to mill cards.