formidable

adjective2

1. •
   Causing fear, dread, awe, or discouragement as a result of size, strength, or some other impressive feature; commanding respect; causing wonder or astonishment.

   • […] and I am more afraid of those people than I am of Anytus and his colleagues, although they are formidable enough. — 1954
2. •
   Difficult to defeat or overcome.