For one thing Greivous Wounds really mess up Darius. If you have ignite on your team, they should ignite him. Or consider getting Mortal Reminder to decreasing the healing. And let's be real most mana based mid laners nowadays get morellonomicon anyway which helps as well.
As far as focus, if your an ADC, you really don't have the capability of diving past a Darius. Instead kite him, dodge him skill shots and rely on your team to peel for you. If they don't peel for you, ya it'll suck, but that's job (provided they are playing who can peel) so if you lost team fight at that point, don't blame yourself.
If you're a Marge let's be real, unless you are Kayle or Azir (with their sustained dps) you aren't going to burst down Darius. Use whatever CC you have to help peel but save your full burst combo until either you can find the right targets or Darius is already dead.
If you are a tank, just peel him off of your carries. Take Maokai for example. He is very good at peeling champ likes Darius due to the low Cd on his Q and W. A lot of times with five fighters like Darius it's better to simply peel for your team then to straight up engage on their backline (as Darius will kill a carry faster than Maokai will).