Formal:
* The announcement of the company merger completely flabbergasted the employees, who had no prior knowledge of the plans.
* The magician's incredible illusion flabbergasted the audience, leaving them speechless with wonder.
Informal:
* I was flabbergasted when I found out my neighbor had won the lottery.
* My jaw dropped and I was flabbergasted when I saw the price tag on that dress.
* "You're going to do what?!" she exclaimed, flabbergasted by his impulsive decision.
Figurative:
* The sheer audacity of his proposal flabbergasted the entire committee.
* The beauty of the sunset flabbergasted her with its vibrant colors.
Remember, "flabbergast" implies a strong sense of surprise and astonishment, often bordering on bewilderment or disbelief.