"Knocked flat" is an expression used to describe someone or something that has been knocked down completely, leaving them flattened or powerless. However, there are many other synonyms to express the same feeling, such as "flattened," "crushed," "dejected," "defeated," "devastated," "beat," "demolished," "annihilated," "wiped out," "overpowered," "demoralized," "vanquished," "trounced." All of these words help to convey the sense of being completely overpowered or defeated in one way or another. Whether it's in a literal or metaphorical sense, these synonyms provide various options to communicate the same message.