what would you suggest then? (honestly) id like suggestions if you have some. even if my team focuses him its difficult to kill him.
He is a squishy who runs around looking for ways to do tons of damage with very little cc often dependant upon his team to protect him yes? He tends to survive by healing like all other carries ja?
Carries tend to be a major source of damage on any team and tend to outmanuever many champs by healing evading juking and dealing tons of damage fast. The best champs for killing them such as assassins, heavy damage CC mages, fighters, bruisers, and anticarries are an important part of any team but it means nothing if everytime you teamfight everyone just throws everything they got at whomever they can.
Give team members responsibilities. AP yi is only strong because his job is obvious. Press R and Q people who might die. rinse repeat. He plays like a really poorly made assassin with a cute ability reset gimmick. He still functions like a carry but with a rather awkward way of healing. If you've ever seen fiora or vayne with bloodthirster youll notice that they too can tank your whole team and they too get melted the second they get CC'd. Most support champs can CC him and have time to do so as they have no business in the front lines of the battle anyways. Focus yi, kill yi, consider the enemy carry killed and celebrate.
If you have an assassin you are in especially good luck since most assassins can dump on yi and kill him as he tries to meditate or as soon as he comes out of alphastrike. Sometimes before.
In lane, do not treat him like a mage. He builds AP but he is not a mage. Poke him if you can and force him to blow his pathetic manapool. Out of all mana champs he possesses the 7th most bad manapool in the game and must save for his heal. I forget what it looks like but make sure if he seems to have his passive up that he doesnt get a chance to alphastrike on top of you. He cannot burst without items so hp pots or whatever sustain you have will be satisfactory to last against him if you need to reach level 6 or something. He must push lane to alphastrike 1 minion amongst many. Let him, he has no escapes. Freeze the lane and call for ganks or just attack him if you can keep up with him. Just remember that he's a carry with high move speed and an AS and damage steroid not that you should be in melee combat with his non AD building self ever.
Dont worry about chasing him if he seems determined to escape, he'll probably get away and is too squishy to do much without HP. BTW be sure to check his stats for unusual levels of MR and all that depending on who he has expecting to lane against. Especially if he's using MR per level or something.