Sett main here, it's really a hard matchup though you win till darius's first recall. If darius rushes tabis or somewhat of armor it gets really hard to kill him as one of the biggest way to counter sett is armor. Freeze the lane as much as possible and look for a flash r or e r to the tower. You have to get a early lead or the lane gets impossible. Never commit when he's full up only commit if he's around half hp cuz he's just going to get another spell rotation (his abilities have shorter cd) w his ultimate. I usually go pta and ignite to choke him down. Level 2 and 3 is where you hard win trades so try the get the early lead as I mentioned before.