Rengar will try to power farm early so he can get his and start ganking more effectively. Try to delay this power spike by invading him and stealing away his camps.
Try to place vision inside Rengars jungle so you know where he is at all times. If you spot him on the top side (pre-level 6), try to invade the bottom side jungle and take his camps. Alternatively, you can gank the bottom lane.
Rengar will look to proactively gank whenever there is a low target as well as whenever his is up. Try to counter gank if possible and always ping your allies if they are low. Do not under any circumstances give Rengar free kills as he will take over the map.
Rengar is an assassin and will look to take down targets that are alone. Do not split up as you will help him succeed in his objective of taking down lonely targets.
Fortunately, you can always tell when Rengar is going to engage with his . As soon as you see him engage, try to blow him up so he is unable to one shot a carry.
In team fights, Rengar will try to take down a immobile or squishy target. Regardless of who you are playing, try to get close to one another and protect each other to prevent him from jumping in.
Rengars main power spike is at level 6 because he is able to look to gank and assist his allies. Once he is 6, place vision around your lane so you can back away as soon as you see his animation.
In lane, Rengar has a power spike when he has 4 stacks on his Ferocity bar. Once you see he has all the stacks, back away and wait for him to consume them.
Whenever Rengar has his up, he gains a lot of assassination potential. Try to keep an eye on his cooldowns so you do not become his next target.