Beledros Witherbloom in exile (exiled by Agatha's Soul Cauldron).
All other permanents on the battlefield.
Urza's Saga has two lore counters on it.
Lands you control except Urza's Saga can tap to produce at least ({2} magic symbol) .
The number of artifacts you control plus Arwen's toughness is equal to or greater than 9.
Steps
Activate any number of lands except Urza's Saga by tapping them, adding at least ({2} magic symbol) .
Activate Urza's Saga's second ability by paying ({2} magic symbol) and tapping it, creating a 0/0 Construct artifact creature token that gets +1/+1 for each artifact you control, and that enters the battlefield with a number of +1/+1 counters equal to Arwen's toughness.
When the Construct enters the battlefield, Trostani triggers, causing you to gain at least 10 life.
Activate the Construct using Beledros Witherbloom's ability by paying 10 life, untapping all lands you control.
Repeat.
If other lands you control can tap to produce at least ({3} magic symbol) , this combo nets infinite mana.