Picking Zhonya's makes me think you're playing a mage which means unless you're playing Kayle or Azir (might have missed a few) you're gated by this thing called
###cooldowns!
However, then you have Yi, which... after a little bit of help from items, can excel in something called
###sustained damage!
This is why Zyras or Brands may not stick in fights forever. You're gated by cooldowns, which means you should be seeking the aid of either a teammate or a nearby tower to help damage the yellow speedster. Yi gets scared the more people that show up.
Mage's damages come in specific windows thanks to cooldowns, whereas Yi and ADCs can literally auto forever until you're dead. If the other comments are correct and you were playing Lux... well... she's definitely cooldown-dependent.
I mean, I still think he shouldn't be getting these buffs he keeps getting but when spreading the knowledge of being gated by cooldowns is important to me. :c