Holding priority, cast Dualcaster Mage by paying its mana cost.
When Dualcaster Mage enters, it triggers, copying Presumed Dead, with the copy targeting Dualcaster Mage.
Resolve the copy of Presumed Dead.
Holding priority, activate Woe Strider by sacrificing Dualcaster Mage.
When Dualcaster Mage dies, it triggers, returning it from your graveyard to the battlefield.
Repeat from step 3.
Resolve all Woe Strider abilities from step 5.
This clever loop turns Woe Strider into an infinite sacrifice engine by exploiting the delayed return trigger granted to Dualcaster Mage. When Dualcaster Mage enters the battlefield, its ability copies Presumed Dead targeting the Mage itself, setting up a permanent safety net for when Woe Strider sacrifices it. Every time Dualcaster Mage hits the graveyard, Presumed Dead's deferred trigger immediately brings the Mage right back to the battlefield, where it safely repeats the process and lets Woe Strider churn through your library with infinite scry triggers.