Dual Nature and Cloudstone Curio on the battlefield.
({11} magic symbol) ({R} magic symbol) ({R} magic symbol) plus enough mana to pay for commander tax, if applicable available.
Steps
Cast Leashling by paying ({6} magic symbol) .
When Leashling enters the battlefield, Dual Nature's first ability triggers, creating a token copy of Leashling.
Cast Etali either from your hand or from the command zone by paying ({5} magic symbol) ({R} magic symbol) ({R} magic symbol) plus commander tax, if applicable.
When Etali enters the battlefield, it, Dual Nature's first ability and Cloudstone Curio trigger.
Resolve the Cloudstone Curio trigger, choosing not to return anything to your hand.
Resolve the Dual Nature trigger, creating a token copy of Etali.
Put the token Etali into your graveyard due to the legend rule.
The token Etali and Cloudstone Curio trigger.
Resolve the Cloudstone Curio trigger, returning Etali from the battlefield to your hand.
When Etali leaves the battlefield, Dual Nature's second ability triggers, doing nothing.
Activate the nontoken Leashling by putting Etali on top of your library from your hand, returning Leashling to your hand from the battlefield.
When the nontoken Leashling leaves the battlefield, Dual Nature's second ability triggers.
Holding priority, activate the token Leashling by putting Leashling on top of your library from your hand, returning the token Leashling to your hand from the battlefield.
Resolve the Dual Nature trigger, doing nothing.
Resolve the token Etali's trigger from step 8, causing each player to exile cards from the top of their library until they exile a nonland card, and allowing you to cast the nonland cards without paying their mana costs, with you casting Leashling from your library.
When Leashling enters the battlefield, Dual Nature's first ability triggers, creating a token copy of Leashling.
Resolve the nontoken Etali's trigger from step 4, causing each player to exile cards from the top of their library until they exile a nonland card, and allowing you to cast the nonland cards without paying their mana costs, with you casting Etali from your library.
Repeat from step 4.
Results
Cast all nonland cards from each opponent's library.