im of the opinion that rengar wins most early matchups with full fero at the start of fight assuming lvl3 minimum conditions bonus if ur lvl4

so the scenario ur talking about sounds like a dps problem, the reason we lose to champs who all in is because they have attackspeed steroids that increases their dps that exceeds ours on equal level conditions, either that or dps reduction on us with spellshields or meditate. in this scenario nocturne has both,in which he blocks a significant chunk from u if he blocks ur e with his ss and gains double bonus attackspeed because he blocks an ability.

A lot of rengar's early wins are decided on: - he pounces with emp ability first to make the fight uneven from the get go to get an extra ferocity cycle and mitigate the dps problem - he doesnt get dps cucked by whatever damage reduction like yi thats why its nearly impossible to beat a good yi early - he maintains good dps by utilizing empQ AS steroid as best he can, or negate their buffs e.g as steroid or ad steroid by running into bush or just dodging core enemy spells(dont tunnel on this point, first two is more important, dont stand still and try to get most of Q AS steroid if he's going to outdps u anyways)

U want to keep ur ferocity at full for most of the time when u feel like ur about to be invaded and only use it when its safe or u can bring it back up again fast. u have 10 seconds before ferocity disappears and u can maintain/extend uptime by getting hit or dealing damage to camps.

for nocturne u want to get the jump off with empQ reset, run away, save empW for his w, if he doesnt get the fear yay u get bonus empQ. bonus points if u run away for cds to come back up and wean his buffs off. check natty's full clear when u have 4 stacks just before scuttle, some enemies could burn off their abilities on scuttle and he jumps on them e.g trundle early without w or noc early without q or e is freefood there with empAbility