You want to get early kills. Therefore, you want an engage supporter, who enables you to hit your spells and provides all-in dmg. If you got an early kill, you are always able to oneshot an enemy with lvl.6 and your support. For a solo oneshot you need at least Ludens or to be very fed early on. Lvl. 9 gives you a similar dmg boost.

You get outscaled by the most adc's in teamfights. You do not need peel, because once you did your combo, you are useless. Therefore, peeling you makes no sense.

You want to pick Syndra, if you have at least two ad dmg dealer. Additionally, you need a dps champ, if they got more than one tank. Of course, your support has to pick a champ with a good synergy. If he goes for e.g. Janna, you do not pick Syndra no matter what.

Get early kills, so you can oneshot the enemies. If you are behind, you are a cc bot. Look for catches. You still oneshot adc's tho.

Standard dmg combo:
Prepare a sphere > W it > Q > E > W2 > R

you can also take minions for your W, but this means you have one sphere less for your R.

If enemies are in range of E, you can also E > Q instead of Q > E, which results in a quite longer cc and a safer hit for you stun.

For full dmg stack spheres, so you r deals more dmg. You can expand the duuration of ashere with W.