Well there are a lot of mid lanes that beat ap yi, but as you get more experienced with yi the games will get easier with them countering you or not. You can build any way you like but this ap yi build i find is the best due to the fact you can do damage after the alpha strike and don't just ks.

When i was playing with some friends i went yi and was originally going to jungle no one was paying attention and we had solid ad for a team, so i quickly switched over to ap runes and masteries set my smite to ignite and was ready to role. I then decided to figure out a new build since it was draft. When i came made this build i had no clue about my late game and just was amazed by the results. so i tried it again the next game and did even better, so on and so forth. My friends even use this ap yi build for ranked due to the fact you do alpha strike damage you can heal to full health you have attack speed damage off auto attacks and can backdoor if needed to. (thats what every ap yi wishes to do).

As it may be hard to 1v1 someone in a fight this yi build is able to help secure a kill. Yi is also squishy with this build but can heal back up and needs to play smart. When you get used to this build team fights will become easy as you alpha strike their entire team then go for the carries with your team if they start to focus you flash or ult out.

Hope this guide helps.

Pros: Cons:

A ton of damage early Squishy before level 10
Great for farming Easy to run oom until tear of the godess
Out sustain any mid champ Can only do alpha strike damage until wits end
Late game backdoors Squishy late game in teamfights
Late game Damage Hard to 1v1 people