He can be countered by range, but that requires taking ranged champs which are less viable than bruisers. IE you can counterpick him in ranked, but it's risky in normals. And he does have a decent gapcloser/grab, so sending a squishy against him is definitely a skill matchup.
He can be countered by spellshields/kayle/trynd. Timing's not easy, for sure.
Saying he can be countered by focus is dumb. He builds full tank, 99% of the time. Of course 3 of you can focus and kill him. But if you do, you just used all your time, and all your skills, killing the tank (and took some damage doing so, due to his AoE). His team then wipes you for an easy 1v3.
That's the problem with Darius. Too much damage when built tanky (or even fulltank)