Senna is a broken champion, but ONLY when you get go Fasting. Make sure to get a support duo or ask your support in champ select if they even know what fasting Senna is. Don't try to go ADC Senna and farm because you lose a lot of value from your passive, and don't try to go support Senna since you are extremely vulnerable going double ADCs botlane.

Senna is a really good pick, but sadly you need a Fasting support, like mentioned above, get a duo, or ask your support in champ select if they wanna do fasting. Avoid all-in lanes because you are very vulnerable to them.

Remember to go Sickle and telling your support to go Doran's Shield
Let your support farm, and focus on poking the enemy to get souls and money with your Sickle. Try to trade with them eveyrtime you can, but avoid getting caught out by CC because you will lose that trade. Be sure to heal your duo back up if they're getting range abused, and try to get them to stop poking them down by pressuring them off when they try to do.

Group with your team! Try to get behind your team and when fighting, using the Q for healing them and damaging the enemy at the same time. Don't use all your Q's for damage, remember to use them for your team too. Try to make picks by slowing people with Glacial and using W after. Don't overstep too much, you are extremely squishy and will blow up quick.

You have so many souls you can attack them from fountain. Infinite scaling champion, yay! The longer the game goes, the stronger you are. At this point somebody has to screw up hard for you not to carry. Abuse your range to attack them without them being able to retaliate, and win!