You control a land that enters the battlefield untapped and can tap to produce at least ({GU} magic symbol) .
Notable Prerequisites
You have at least five permanent cards in your graveyard.
Steps
Activate The Core by tapping it, adding at least ({G} magic symbol) ({G} magic symbol) ({G} magic symbol) ({G} magic symbol) ({G} magic symbol) or ({U} magic symbol) ({U} magic symbol) ({U} magic symbol) ({U} magic symbol) ({U} magic symbol) .
Activate a land that enters untapped by tapping it, adding at least ({GU} magic symbol) .
Activate
Oboro Breezecaller by paying ({2} magic symbol) and returning the land that taps for ({GU} magic symbol) from the battlefield to your hand, untapping The Core.
Activate Gretchen by paying ({2} magic symbol) ({G} magic symbol) ({U} magic symbol) , causing you to draw a card and put the land from step 3 onto the battlefield.
Repeat.
Notes
You should add the opposite color of mana that the land that enters untapped produces when activating The Core. So if the other land produces ({G} magic symbol) , you should add ({U} magic symbol) via The Core, and vice versa.