When Lumbering Battlement enters the battlefield, it and Preston trigger.
Resolve the Lumbering Battlement trigger, exiling Fiend Hunter until Lumbering Battlement leaves the battlefield.
When Fiend Hunter leaves the battlefield, its second ability triggers, returning any cards exiled with it to the battlefield under their owner's control.
Resolve the Preston trigger, creating a copy of Lumbering Battlement that is a 0/1 Illusion.
When the token Lumbering Battlement enters the battlefield, it triggers, exiling the nontoken Lumbering Battlement until the token Lumbering Battlement leaves the battlefield.
The nontoken Lumbering Battlement leaves the battlefield, returning Fiend Hunter from exile to the battlefield.
When Fiend Hunter enters the battlefield, its first ability and Preston trigger.
Resolve the Preston trigger, creating a token copy of Fiend Hunter that is a 0/1 Illusion.
When the token Fiend Hunter enters the battlefield, it triggers, choose not to exile a creature.
Resolve the Fiend Hunter trigger, exiling the token Lumbering Battlement.
The token Lumbering Battlement leaves the battlefield, returning the nontoken Lumbering Battlement from exile to the battlefield.