Target the damage dealers FIRST
I played a game against a Lee sin, Olaf (both full tank), Annie support, a Corki and a Syndra. On my team was Darius, Lissandra, Leona, Kalista and Udyr. We lost team fight after team fight because no one on my team would go after Syndra. There is no reason someone like Syndra should not be blown up by Leona and Lissandra. Both of the have long range engage and had CC to lock her down but everyone was going after the full tank Lee Sin.
If the other team has a immobile high damage carry like Syadra, unless you have no way to get to her or she has crazy peels, there is no reason she should not be target number 1.
In brief, Lissandra or Leona should have just CC Syndra to death which never happened.