Activate Tawnos' second ability by paying ({1} magic symbol) ({W} magic symbol) ({U} magic symbol) ({B} magic symbol) , tapping it, sacrificing two artifact tokens and exiling Great Furnace from your graveyard, causing you to create a token of Great Furnace.
When the Great Furnace token enters the battlefield, Retreat to Coralhelm triggers, untapping Tawnos.
Activate the Great Furnace token by tapping it, adding ({R} magic symbol) .
Caged Sun triggers, adding ({R} magic symbol) .
Activate Tawnos' first ability by paying ({2} magic symbol) and tapping it, creating a copy of the Great Furnace token and causing you to mill two cards.