Blitzcrank

Blitzcrank is one of the hardest counters to the Guard Cat due to his ability to remove all shields from a target, his hard engage by hooking your tether, his massive amounts of CC, and his surprising damage output. Guard Cat is fairly safe on her own, but it's hard for her to do her job of keeping a carry alive against any of the hooking/all-in champions.

Leona

Nautilus

Pyke

Pyke Ult: "That's a nice shield you got there, be a shame if I executed you through it"

Alistar

Alistar isn't as bad as the above champions in terms of the Guard Cat, but his early game engage and lane sustain through his passive can make laning against him hard.

Thresh

His all in is dangerous against any ADC, and his lock down can make it hard to protect your carry from the trading of the enemy carry. As well, Yuumi has no real way to peel enemies off her until she reaches 6, and that peel can be sketchy at the best of times. At least he can't pop your shields.

Soraka

Her on click silence is a nightmare because of Yuumi's new W cooldown on hard CC, as a silence does count and prevents you from reattaching for 5 seconds. Her lane sustain is also insane and her healing can be hard to match up against for a passive support like Yuumi. Don't be super aggressive against Soraka until you and your ADC have poked her down.