Master Yi is way over powered
in defensive curl.
and starting to build 
Master Yi melted me in seconds. 1800 wuju style true damage. Teammates beating on him, he just lifesteals back all damage done to him. Any CC thrown at him and he uses alpha strike to avoid it. Any large nuke thrown at him and he meditates to reduce it to nothing. There was nothing we could do. Every fight the Yi just moves in and penta kills us while melting me easily.
There is no way you can tell me this is in any way balanced. The ult lasts way too long. There is no reason to be doing true damage instead of physical. Alpha strike does not need 600 range. A melee champion should be just that, melee. Meditate should reduce defenses, not increase it.
This champion is completely toxic to the game and needs major nerfs.

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and now you need to send 3-4 man to stop him and use all your CC and sums to kill him. Meanwhile their team can take any objective for free and push down the other towers.
Lets say you chose to engage their team 4v5 while yi splitpushes. You manage to kill their carries and the fight is going well, but you cc is down and you entire team is low... Well yi already took the inhib.. and now he is running at at you, pressing q, aa, q and you entire team has grey screen...