
Patch 7.3a

Favorable matchup
Kennen can limit Darius with ranged attacks and a stun ready to go. The condition is keeping distance and his third ability: going in to apply a mark can hand Darius the pull and the long fight he wants.
Kennen's abilities and empowered attack let him stack marks up to the stun. Threatening that crowd control makes it harder for Darius to chase without paying for it. Not all of your basic attacks apply a mark, and the active of your second ability needs a mark already applied.
His third ability gives speed to keep distance, but it doesn't make Kennen untouchable. Keeping it available lets you get out before the pull and avoid the blade of Darius's first ability, which is the part that heals him.
Attack when Darius walks up to last-hit a minion and your third ability is available. If he misses the pull, you can add pressure; back off before he stacks Hemorrhage and turns the trade into a chase.
As Darius, use the wave to make the shuriken harder to land and don't give up health chasing every basic attack. Press when Kennen spends his third ability or lacks the energy to respond. If you land the pull, don't let him walk away for free, but don't chase through a huge wave or toward his jungler.

Execution matters
Jayce can wear Darius down in cannon form and break up an engage with the hammer knockback. It demands managing forms and mana: leaping onto a healthy Darius with no way out turns your range advantage into his best-case scenario.
The cannon shot fired through the gate lets you contest last hits from afar. Minions can intercept it: you need an angle, not to fire repeatedly into a full wave hoping all the damage reaches Darius.
The hammer third ability knocks Darius back to break contact. It's a useful exit if he closes in, but it doesn't clear Hemorrhage stacks. If you spend that knockback for damage and leap back in immediately, you're stuck in a long fight with no separation.
Add pressure when Darius misses the pull and you have mana and the knockback available. Save the hammer engage for a weakened opponent with a clear retreat, not for starting from scratch against his wave.
As Darius, let the wave intercept his shots and save health for the window when Jayce has no knockback or mana. If he comes in with hammer and no way out, force contact and avoid wasting your axe blade against his mobility. Don't run after him at the cost of the whole wave.

Execution matters
Baron lane Vayne can punish Darius with range, true damage and Condemn. It's a demanding, fragile pick: if she gets pulled with no way out, the advantage disappears. You also have to make up for the frontline your team loses.
Condemn can push Darius away and Tumble helps you reposition before the pull or the axe blade. They're no excuse to walk into his range. If he has already caught you and both tools are spent, Vayne lacks the durability to take his full sequence.
Silver Bolts hits deal max-health true damage when you complete the sequence on the same target. That threatens a Darius stacking health, but switching targets and dragging the trade out until he has five stacks can turn the fight against you.
Attack when Darius goes for a last hit, with Condemn available and space toward your side. If he misses the pull you can contest more basic attacks, but back off before he completes Hemorrhage or his jungler arrives.
As Darius, look for an engage from a brush with denied vision and wait for Vayne to spend Condemn or Tumble forward. Don't chase down the whole lane eating basic attacks. Keep a favorable wave and coordinate cutting off her routes: forcing a fight where she has no space is worth more than contesting her range head-on.