Lux

Don't get hit by her snare. Stay back and react once she focuses your ADC, by Q+W+Qing their ADC in return (this stops the engage and gives your ADC time to reposition). Lux is good at pushing the wave into your tower, meaning more opportunities for your Jungler to gank. You can walk up, E+Q+W+Q Lux since she has no escapes and secure an easy kill. The hardest part is dodging her snare to make the engage actually possible. To counter her pushing power, use your Q on the three melee minions at the front of each minion wave, then help your ADC auto them down. Slow down their push and try to freeze the wave under your turret so your jungler can gank more reliably. In teamfights, you have to react quickly with your ultimate to save the target she's bursting down. Never get caught out yourself; stay behind your team and E your bruisers/tanks into her.