Changeling Berserker enters the battlefield, triggering itself and Molten Echoes.
Resolve the Molten Echoes trigger, creating a token copy of Changeling Berserker.
The Changeling Berserker token enters the battlefield, triggering itself, exiling the nontoken Changeling Berserker.
When the nontoken Changeling Berserker leaves the battlefield, it triggers itself, having no effect since no card was exiled.
Resolve the Changeling Berserker trigger from step 2, exiling the token Changeling Berserker.
When the token Changeling Berserker leaves the battlefield, it triggers itself, returning the nontoken Changeling Berserker from exile to the battlefield.