Start with a doran blade or doran's shield if the enemy have an hard poke or vs Irelia or panth , but doran blade can be the best choice for all the situations , buy min. 3 dorans ( buy 4 if u are back or the enemy have true dmg ) rush tiamat ( no reason to finish in hydra now ) get lw for armor pen , Youmumu for more armor pen , a little bit of critical strike and some movement speed bonus when u ult , finish hydra for life steal , buy bc for armor pen , cd , life and dmg , and the last situational item , trinity is the best choice if u have 20 kill at 30 min , or u want to delete the world.
The 3 or 4 dorans need for ur roam , u will deal tons of dmg , trust me.

Level 1 : try to stack 5 ferocity points hitting minions and the enemy champion , using brushes.
Level 2 : get bola strike and with 5 ferocity jump on the enemy with the full compo : prepare q , jump , when u are flying use the e , and then another q , aa , ignite , and u get the first blood.
levels 3-5 : farm and kill the enemy laner
Level 6 : Ur laning phase is an optional , when u get lvl 6 roam in ALL the lanes , the enemy mid laner is pushing ? GANK AND DELETE IT , the enemy bot lane is pushing ? GANK AND GET DOUBLE KILL.
level 6 -18 : roam and kill , go in the enemy jungle and kill , kill all the alone champs or if u are feed go in 1 v 3 , u can do it.
Dont do team fights , u will die 70 % if they have alot of cc's , try to kill always one enemy champ for dont fight.

You can use all the skills when u are jumping , ex jump and when u are flying press e.

Rengar combo for insta burst is : R- YOUMUMU , PREPARE Q , JUMP , E WHEN U ARE FLYING , HYDRA , Q , W = KILL

Dont fight without brushes

Abuse of ur skills , u need always 5 stack for all the situations.

Use the r when u are sure u will get a kill , the cd is really long.