When Phantom Beast becomes the target of a spell or ability, sacrifice it.
As insubstantial as fear, and as dangerous.
Official Rulings for Phantom Beast
8/15/2010 : If Phantom Beast becomes the target of a spell or ability, Phantom Beast's ability triggers and goes on the stack on top of that spell or ability. Phantom Beast's ability will resolve (causing it to be sacrificed) first. Unless the spell or ability has another target, it will then be countered when it tries to resolve for having no legal targets.