How to deal with a good Rengar as Janna? Had a rengar get insanely fed because our jg was afk. This Rengar mid/late game one shotted me and my miss fortune ADC. Now I was Janna and I figured my ult and shield would be perfect counter, but he'd either go for me or my adc, in both cases when I used up my ult or shield he'd just come right back at us. Basically was one shotting us and if I used my ult then it was only a temporary setback. Basically everytime he ulted I knew either me or my adc would die. How do I prevent this? 

I warded too, but I'm looking into how to avoid getting ulted by him, or what to do in such situation. 