Example : A formidable opponent.
The challenge was formidable.
Had a formidable array of compositions to his credit.
The formidable army of brains at the Prime Minister's disposal.

Example : The formidable prospect of major surgery.
A tougher and more redoubtable adversary than the heel-clicking, jackbooted fanatic.
Something unnerving and prisonlike about high grey wall.

Synonyms : redoubtable, unnerving