Rengar can hop over walls using Unseen PredatorUnseen Predator. Place a ward next to a jungle camp and get inside a brush over the wall, then simply attack the monsters. However, make sure they're inside your circle. Also, when your Bonetooth Necklace reaches 9 Trophies, use the extra range on your passive to make this wall-hopping easier.
Whenever Rengar turns invisible using Thrill of the HuntThrill of the Hunt, he gains a point of Ferocity every 0.75 seconds. Keep that in mind because it can turn the tide of a battle, increasing one of three things about Rengar: his damage, his utility or his self-healing ability.
Some might argue that Savagery is a one-point wonder skill, as its scaling is based on Rengar's total attack damage and thus remains powerful even at Rank 1. It is also difficult to keep opponents in range to fully utilize the bonus attack speed, unless allied crowd control abilities are strong enough to keep foes locked down.
Maxing Battle RoarBattle Roar first gives you a good burst of AoE damage, a high defense boost for a few seconds and allows for generating ferocity faster with cooldown going down. Consider it over maxing Savagery, when jungling or playing defensively.
Thrill of the HuntThrill of the Hunt is a great tool for escaping ganks and other nasty situations. Since it is not delayed by damage you can immediately go in stealth. When you successfully fade into stealth try to take an unusual path to avoid being caught.
Rengar is a burst bruiser off-tank, relying on mobility and disabling the enemies and dealing massive amounts of damage in a short burst. Therefore, he is meant to be built with items that help him achieve this.
Bonetooth Necklace is a very cost efficient item even without the trophy stacks. At level 18 it offers 36 AD (2 per level) for a meager 800 gold and should stay a part of your inventory unless the spot is required for something stronger, later in the game.
Bonetooth Necklace should be bought always, if for whatever reason you decide to get another item later in game, selling it refunds 560 gold - net gold lost is only 240 gold (less than a Elixir of Fortitude).
Rengar has higher than average base attack speed (0.675). He also has good AS per level scaling, totaling 48% bonus AS at level 18. His double Empowered SavageryEmpowered Savagery allows him to gain another 150% bonus AS for nearly 3 seconds. High base attack speed requires exactly 270% bonus AS to hit the hard cap. His kit can grant him up to 198% bonus AS, which in such case leaves only 72% bonus AS to go via masteries, runes and itemization.
Build and use bonus attack speed on Rengar wisely - any attack speed that brings him over the 2.5AS cap is wasted.
Since Rengar deals mostly physical damage, buying armor is a top priority. Ninja Tabi and Thornmail are very effective against him since the majority of his damage comes from autoattacks enhanced with Savagery.