I stumbled upon an easy kill for Elder boss. I have no idea if this works in areas which do not have a slope but I wanted to share anyway. So I was shooting fire arrows at him and ran away from him down the slope. I started running back and forth along behind some trees parallel to the coast and he would shoot out his vines of course. But I noticed that when I got far enough away and as I was hiding for a few seconds behind a tree he stopped and didn't move. So I popped out and shoot at him....He still did not move. So I basically found a spot where he freezes and I was able to shoot him until he died! I did this twice.