Lumra will die as a state-based action due to having 0 toughness.
Resolve the
Lumra trigger from step 2, causing you to mill four cards and return all lands from your graveyard to the battlefield tapped.
When the lands enter,
Emeria Shepherd triggers that many times.
Resolve an
Emeria Shepherd trigger that was triggered by a Plains, returning
Lumra from your graveyard to the battlefield.
Repeat from step 2.
Notes
You can return all nonland permanents in your graveyard and library to the battlefield if you control there are two or more Plains between your graveyard, library and battlefield. To do this, have all triggers from
Emeria Shepherd target desired permanents. Then, resolve the Plains triggers first to return those cards to the battlefield, causing all other triggers to fizzle on the stack.
Results
Infinite LTB.
Infinite ETB.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite self-mill.
Infinite lifegain triggers.
Infinite lifegain.
Infinite landfall triggers.
Put all permanent cards from your graveyard and library onto the battlefield.