"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"

"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"