Formidable or tough — describing an opponent or problem that is strong and not easy to overcome or deal with.
今回は手強い相手だから油断できない。
This time it's a tough opponent, so we can't be careless.
このパズルは想像以上に手強い。
This puzzle is tougher than I expected.
あの選手は年を取っても手強さは変わらない。
That player remains formidable even as he gets older.