Trundle

All-inning a Trundle is never a good idea, and you can't counter him with crit builds either. Avoid him at all costs late game, try to trap him with teammates and CC early.

Rammus

Counters basically every Rengar build known to man. If you do find yourself in a matchup vs. Rammus, utilize your max ferocity E to keep him from getting on top of you.

Shyvana

This champion does crazy damage for no reason and is basically unfightable at every stage of the game without help. Try and catch her greeding for dragons if you have bot/mid prio.

Skarner

If you do happen to face anyone who actually plays this champ, he does counter you pretty well. His tankiness and R will screw you over. Avoid him at all costs in team fights.

Nocturne

Late game he isn't as much of a problem, but Nocturne can abuse you early and mid game. His stickiness in combination with his bruiser build make him difficult to get away from if he wants to fight you.

Wukong

Just like Nocturne, early and mid game he is a menace. His R and his W make it hard to get away from him. His weakness is CC so wait for Wukong to use his E first and then throw a max ferocity E to root him.

Darius

Udyr

Udyr's early game is insane and if the pilot is good enough, he will abuse it to the max. His Bear Stance will screw you over if he stuns you with it, so save your max ferocity W to get out of that.

Warwick

Same story as Udyr, his early game is very strong. However, Warwicks are usually very predictable as they like to gank where your laners are low health because it procs his W passive (movement speed). You can use this to your advantage and countergank him.

Kindred

Kindred's playstyle is similar to Rengar's, relying heavily on XP leads and snowballing to win games. It's better to take it to them than let them take it to you, so be aggressive, but be careful of Kindred's E and use max ferocity W to remove the effects.