Activate Warren Soultrader by paying 1 life and sacrificing the persist creature.
When the persist creature dies, its persist ability triggers if it didn't have a -1/-1 counter on it, and its undying ability triggers if it didn't have a +1/+1 counter on it. Luminous Phantom also triggers regardless if it's on the battlefield.
Resolve the Luminous Phantom trigger, if applicable, causing you to gain 1 life.
Resolve the persist trigger, if applicable, returning the creature from your graveyard to the battlefield with a -1/-1 counter on it.
Resolve the undying trigger, if applicable, returning the creature from your graveyard to the battlefield with a +1/+1 counter on it.
When the creature enters, Lunarch Veteran triggers if it's on the battlefield, causing you to gain 1 life.
Resolve the Warren Soultrader ability, creating a Treasure token.