Against sett, Expect Sett to flank in a team fight or use Hexflash to get over a wall and catch someone out with his E or Ultimate R. 

Avoid grouping to closely as it may allow Sett to get a multi-person E off which will help his team win the team fight. Splitting up slightly will be better for you in the long run.

Do not walk too far forward when sieging an objective as Sett may use his Ultimate R to pick up the closest champion and launch them into their allies behind them. If you’re grouped together and in line with one another, it will be harder for him to do this. Against sett, Expecting that Sett will snuggle in a team fight or use Hexflash to cross a wall and catch someone with his E or Ultimate R. Avoid regrouping closely as this can allow Sett to get a multi-person E that will help his team win the team fight. Slightly split up will be better for you in the long run. Don't walk too far by sitting a goal as Sett can use his Ultimate R to pick up the closest champion and throw them into their allies behind them. If you are bundled and online with each other, it will be harder for him to do so.