In the situations in which rengar has low stack of tooths or simply cannot burst he should be built similar to other tanky dps (Mostly damage sponge and sustain with enough damage to hurt against low defense targets).
In the case where you're falling off you should shoot for the heart of gold quickly for your GP5. Depending on your lane opponent or where your damage is coming from you will want to grab a warden's mail early or get your merc treads. Grab a phage for a bit more health and damage plus an additional slow to your bolo.
Your end build should be Frozen mallet, Randuins Omen, Merc Treads and Last Whisper as your core items. This plus your necklace should provide a lot of damage, CC to hamper a retreating target and CC to hamper the entire team. Depending on if you have a lot of damage coming in from magic or not you may want to pick up either a wits end, maw of modius, Guardians Angel, Force of nature or Bandshee Veil.
Team Fight Strategy:
Your job is simple in this case. Stand amongst your team, wait for an initiate (Don't get caught by AoE) and as soon as the fight starts, Ult and land on top of their AD carry. Pop your randuins as soon as you do and go all out on that AD carry. Your reduced damage shouldn't matter since he's gonna be squishy, if he's stacking armor your last whisper should cut it down, if he's not it's still gonna help (If they have more than 30 armor the last whisper does more damage than an unstacked bloodthrister). Your warden's mail will reduce his attack speed for attacking you and the armor should keep you alive.
Your job in every fight is to stick to that AD carry as a damage sponge that can kill him. He will not be able to just ignore you and you take a fair amount of time to die thanks to your debuffs. Add in exhaust for extra CC on him and lock him down from the fight. He will probably kill you, but by doing this you force their team to focus on a damage sponge thats on top of them instead of your carries. They will be bunched together if they focus you, perfect for your AoE CC and the randuins slow prevents them from splitting up outside of flashes.
That would be my strategy in this case. Use different skills at different times depending on the AD carry (Leap on corki a second time if he dashes away or bolo him with ferocity, etc)