Really? Teostra and Lunastra are so ridiculously easy.. Vaal is pretty easy, but his Miasma is more annoying than Teostra's supernova. And Kushala is just very time consuming for me. And Xeno is actually ridiculously easy for the last Elder Dragon. So Nergigante is the one I have most trouble with, while Teostra/Lunastra are easiest.