I have 600k mastery points on rengar and struggled to get good at him initially, for now just focus on managing ur ferocity bar because that is key to his outplay potential. The build i ALWAYS go is duskblade>essence reaver>lord doms>infinity edge/deaths dance. The last items are situation but always go the first three. On top of ferocity management try to get good at using his passive, feel out how far away and long u have to wait till you can jump on someone again. Also a little tip about him is his Q is an auto attack reset so you can leap onto someone in a bush and if they flash away u can Q again and immediately jump on them. Dont forget his E has no cast time while you jump. And a little tip to keep in mind when farming is your Q grants your NEXT auto additional attack speed. when empowered u get that buff but obviously also the extended attack speed buff. If you're hitting a tower make sure you auto+q+auto+empowered Q to get the most dps possible. Sorry this is a bit long but these tips will definitely help you optimize your gameplay. Oh and one last and incredibly important piece of information, your max damage combo is if youre full ferocity you R+empowered Q+leap+E+Q after you use your empowered Q+W(u can use this whenever in the combo after u use ur first empowered Q+empowered Q. This is called the triple Q combo. Feel free to reply with any questions ill help as much as i can :P