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what counter play is there to a fed anything? you're talking about the extreams of a champion. Fed garens dont die and 1 shot carries but thats fair because he cant run away as well?
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you effectively gimp an entire class making them impossible to push leads because the more the level damage the longer they have to wait to use it.
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reset assassin completely negate this idea as you cannot implement it without effectively just deleting them
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your getting one shot because your not a tank, assassins ( unless fed ) dont just one shot everything, and in some cases they still can't one shot a carry. ]
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there is no need to nerf assassins, because then you should nerf mages as well who do the same job from a 500-600+ range

Getting one shot isnt a problem, 0 counter play is ok when something is so fed kench would be benched.
also you wanna stop fizz? have a tank front line to stop his ult from touching the back line then locking him down if he goes in, or play a mage because banshee's is fucking stupidly effective against him.