Im sry to say so but u cant actually oneshot an enemy with just R. The first thing is that u would have to be lvl 18 and the enemy should be in lvl 10 or less, else he will have enough health to survive the attack. The other problem is ur suggested item build - u would deal more crit dmg with infinity edge that also effects rengars ultimate. Also building attackspeed (trinity force) on rengar is no longer vailable because u want as much ad as possible to make full use of the passive that gives u 15 + 30% bonus ad if u killed every enemy champion. Instead of trinity force u could build the black cleaver, it has phage, a lot cdr, more health for the w and double the ad.