For Senna: You want to play safer early game because of Senna's mana and try not to waste much just go for poke and proc your Grasp as much as possible. You want to look for a combination of your W root with Sett's E pull. Both are really easy to hit so it doesn't matter which order you do it in. You will be doing most of the constant damage while sett is just doing a lot of bursts with his W and his auto's. You have to look how to back up sett when he goes in with his ult and heal him or even ult him to give him even more shields. You and Sett heavily buff each other so even if you make a ton of mistakes you will probably just win because of the champs you picked. A really fun thing my duo and I do is I use my E on Senna and he uses his Q on Sett and run at the enemy champs so fast to look for the pull root combo up close. You should be aiming for 30-40 mist wraith stacks every ~10 mins on Senna.

For Sett: You want to focus on early game cs while relying on your senna to poke them out of lane. You want to push hard always because even if you guys get ganked it's going to but a difficult challenge to kill you. Once you get your ult you could easily push them under tower by just running to them under tower and ulting them out of tower range while your Senna follows close behind confirming kills and cc'ing.

For Senna: Even if every single lane lost you besides yours it doesn't matter because you and Sett can basically 2 v 5. You and Sett do a **** ton of damage so just follow off each other, so just look to support him with your ult and your Q. Most of all you do a **** ton of damage. You have by this point a Frozen Mallet and a Black Cleaver meaning you do a lot of damage and are incredibly tanky with your runes and items.

For Sett: You are only building tank so like you really just have to dish out the damage with your W and just have your Senna do a lot of damage. Try and ult the tanky people into their team for the slam damage. Look for big pulls for a cc train.

For Senna: I really doubt you guys will get past 25 minutes because you and Sett will be so far ahead there's no turning back but if you somehow get past this point you do more of the same just look to keep your Sett up with your ult and Q while also rooting people in a group while dishing out dmg with your auto attacks.

For Sett: More of the same I again doubt you'll get to late game just try to build more health to buff your W.