My thoughts and observations on Rengar
Famous for his 0.03 instant kill and his jump scares, it's no doubt that Rengar is a powerful champion. But after playing as him for a bit, I learned some of his strengths and weaknesses.
Pros:
- Takes out carries quickly.
- Great at securing kills with your team using bola strike.
- Has a decent sustain with empowered w.
- Great snowballing opportunities.
- No mana.
Cons: 1: Cannot kill tanks easily. 2: No escape potential besides his ult. Even then it will rarely help him escape. 3: No aoe cc and little aoe damage. Bola strike will only strike one target.
For Rengar to even be a viable champ, he needs his kill potential. What most want is Rengar to not instant kill, which gives your team the opportunity to stop Rengar from finishing off the adc or whoever they're protecting. The problem is that Rengar is all about kills, and by delaying his kill potential, he is pretty much useless. What defines an assassin is the ability to take out a target quickly and get out. And Rengar sure has the kill potential, but not the escape. LeBlanc is a good example, she has the opportunity to instantly kill her target and get out. Also keep in mind Rengar needs to be really fed in order to assassinate carries instantly.
In order to thwart Rengar, it's very important to play tanks and heavy cc champions. Late game he will struggle to find a target available for him and likely end up getting caught out. Also make sure you get sweeping lens and hide your adc in hidden brushes at a far distance. I would recommend like having 2 or 3 of them. Just make sure that Rengar cannot see any easy targets. Some examples of good anti-Rengar picks would be
For his long supression 
For his stealth
For his global true vision
For being tanky and sturdy
Rengar is hard to play against. It gets especially frustrating when you didn't feed the Rengar, but he got fed off your teammates and then instant kills you. Personally, I think the amount of precautions necessary when playing against him are too much, especially for solo q when it's hard to communicate extremely well and coordinate good positioning.
Let me know what you think
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Malzahar couldn't be instant killed, so his team has the opportunity to cc him down and take him out. If Twitch is too up close to be ripped apart by Rengar, he is too out of position. Twisted Fate will reveal Rengar, giving the tanks on your team a great opportunity to strike and Twisted Fate's stun card will also help.