Well, with Leblanc, if you can get into Q range of them without using W, they are basically dead. Your combo will basically be Q-R-W-E. Throw in DFG in between Q and R.
If you're out of Q range, things get a little more difficult. You won't have your W damage and silence proc, so you'll rely on hitting your chain for sure. The combo then would be W into range, Q-R-E.
For Syndra, you basically want to poke at them with Qs for a while until you see an opportunity to line up a toss-stun (or just a stun if they're really out of position) and ult them. If you really want to master Syndra, you can get more than 5 balls out at once by abusing the throw (pick up a ball and hold it until Q comes back off CD), but Syndra can still 100-0 just fine with 5 ball ults.