What champ counters things like a fed kha'zix?

Don't let him get fed.

Tanks with good peel who are good at sticking near squishy dps units.

Champs with good single target shutdown, and scale well off of armor. This works by both isolating the threat from your team, and countering their AD damage, while scaling well yourself. For example Rammus. Your E+W combo is excellent at stopping AD heavy carries, with a few exceptions, and it allows the squishier damage dealers on your team a chance to do their jobs.

Someone who can burst him before he has a chance to burst you, or someone who has a lot of CC but still one shots people ( )

how can you counter a fed champ who is ahead and is single target burst?

hmmm You dont let them get ahead

every brainless point and click cc champ basically.

for example

A really fed tank.

It's not about what champ, it's about how to avoid the isolated damage. I would start by warding your flank, and putting down a control ward during a team fight. I would also try to stay with your carry if you're support, or if you're a carry head towards your support. Try to catch him out of position, and try to organize a collapse on him.

In lower ELO's the more fed the champ, the cockier the player, he/she will often make mistakes and that is when and how you capitalize on the fed factor.

1. Point and click CC+burst, even if not fed ( )
2. Stunlock into big damage 2-man combo ( + )