Against sett, 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. 

When no team fights are occurring, Sett will put himself in a side lane and try to split push. When he is alone, you could look to either pick him off or start a team fight with his allies when he is unable to help them. Against sett, 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 in setting a goal as Sett can use his Ultimate R to take the closest champion and throw them into their allies behind them. If you are grouped together and online with each other, it will be harder for him to do it. When there is no team fight, Sett will set himself in a side lane and try to divide the thrust. When he is alone, you could look for him or start a team fight with his allies when he is unable to help them.