Well, your first mistake was picking Rengar. Not just because he's Rengar, but because he's a slow jungler, he's vulnerable to counter-jungling anyway. All you can really do is get a Q off, or a W if you're starting that. You won't do any damage and you don't have any tankiness. You have to have your team guard you and your other buff while you wait for spawns. If you have advanced warning and know you can't fight it, rotate out and then take THEIR blue buff if they're at yours. Slow junglers like Amumu and Nautilus are particularly vulnerable to Shyvana and Nunu because they'll just get to the wraith or red camp before the others do.