The start of the game will be the harder part because you only have boots. But when you will go for you phage and zeal the game will becaume alot easyer because you are getting tanky and if you follow the rune that i but yoou should have pretty good ad.

The mid game if succes your starting game by get some kill or have a good cs farm you only have to focus the mage you should kill them pretty fast by using all you spell by starting by you Q. Even in mid game try to farm the more cs you can for get the bloodthirster and the infinity edge the faster you can.

If you succes get a good cs farming and some kill you should totaly own the mage and the carry ad too if they are not to feed and you can get them by surprise by jumping from a bush or using ult.

from lvl six to 16 try keep the ult for escape gank and ambush. But lather when you get 16 that why cs farming is very important to your ult get very low cooldown so you can use it for enter in fight and target the mage and deal to it a devastating Q by using the 5 stack that your ult give you

Rengar is very good for the backdoor because he can nuke tower pretty fast by double Q on it and gain bonus attack speed. Rengar can easily get out of trouble after backdoor because he running 455 when the bonetooth necklace is stack and with his trinity so he can go out very fast if he ult he become invisible and run very fast. If the other team have a yi or another champ that run very fast too you can combine your ult and your ghost for run faster.

Rengar can easily late game solo dragon and be very useful when killing the baron because of is last hit and his nuke damage done by his Q.

If you have a good tank that protect you from most damage and you are able to nuke the bif enemy damager you can take a phantom dancer at the place of the frozen mallet so your auto attack have more chance to crit and you run faster because can't escape you anymore and you can go out fast of combat because of your movement.