You will kill enemies as FAST as you do with the glass cannon assassin rengar that we all love/hate depending on which side you are from that rengar.

You have enough armor from Dead Man's Plate. Enough damage from Titanic Hydra + Last Whisper, so if the enemy has a fed AP, you can sacrifice your Infinity Edge for a Spirit Visage; and your mobility bookts for Mercury's Treads.

Also if the enemy is extremely tanky and you have the curse of being an ALL AD team, then get a Black Cleaver instead of Infinity Edge.

The reason I don't go AD runes is because you rush tiamat, which provides enough AD and AoE damage for insanely fast clear. Armor Penetration is always super good against enemy champions: If they are squishy, you almost do true damage to them. If they are tanky: you penetrate that armor a lot more effectively even before buying armor penetration items (Last Whisper in our case).

Go standard aggressive 21/9 masteries, just keep in mind that whatever you do; you need to keep the 5% CDR. It is really important to have 10% CDR from Runes & Masteries.

ALWAYS rush Tiamat, but only complete the Titanic Hydra after you finish both Dead Man's Plate AND the Sterak's Cage. Then go for your offensive items (Last Whisper & Infinity Edge).

You can go for only Last Whisper as the offensive item and get another tanky item like Spirit Visage, Randuin's Omen, Warmogs... anything really. Because after the Titanic Hydra + Dead Man's Plate + Sterak's Cage + Last Whisper ... well you're a scary fat legendary cat that no one will even bother fighting.