A guide for those who wants to seriously climb the ladder with a unique build than the current meta.

Runeglaive is essential in order to clear jungle creeps and for the 10% cool down reduction.
Ionian Boots for the 10% cool down reduction.
Nashor's Tooth for the 20% cool down reduction and the bonus damage per ap scaling.
This is all needed in order to get 40% cool down reduction and to clear jungle creeps efficiently.
You might have to build void staff if they start building magic resistance against you.
Hourglass is just to keep you alive for 2.5 seconds so that if you use all your skills already and in a bad position then you use hourglass and hopefully your skills are ready to use again.
Deathcap for more Ability Power.

When team fighting you won't be there but will make the fight easier for your teammates because as AP Rengar you are just going to take down the carries making it a 5 v 4. Keep calm and just take down the weak. Make sure you charge up your ferocity in order to deal more damage.

Jungling is going to be difficult at first wave but after going back and purchasing your stalker's blade you will be fine. Start with Q but max W.

One hits carries / Needs to charge up ferocity
1st jungle wave is horrible / fast clearing after getting runeglaive
Teamfights are going to be 5 v 4 / You won't actually be with team

AP Rengar that makes teamfighting easier for your teammates and easy to use. Avoids current AD meta in a unique way. Faster thrill of the hunt and faster burst.