Wait until she hops off to proc her shield, then walk toward her ADC.
As soon as she autos, you and your partner throw autos at her.
She isn't going to peel your ADC on her own, so she has only one move:
Hop onto her ADC.
Knowing this, all you have to do is be close enough to throw your CC at her ADC as soon as she gets the shield.
Alternatively, if you don't have hard CC, just wait until level 6 and one combo the ADC. Her "peel" is only a root, so it won't stop you from getting your combo off. And she can't heal someone who's already dead.
Also her heals are more effective the lower the HP of her ally is... which means she's going to want to save mana and her Zoomies! cooldown by waiting until their health starts to get low.
So while you may not be able to poke them to death, you CAN poke off some of their health as set up for your level 6 wombo. A good Yuumi will not waste Zoomies! charges healing off minor poke because she gets more value out of them if she waits until you go for the kill and then rapidly uses both.