Its strange, on my first play through i blew through alot of bosses with ease, and a few gave me trouble. Then i did a play through another time with a different build, and alot of the bosses i found easy were extremely harder and the hard ones were alot easier. Crazy stuff

If you one shotted cinder without a summon, cudo's to you, im gonna take a guess and say you done a powerful ranged magic build? As one shotting meleeing him would have taken alot of time/patience O_O

The nameless king for me was the hardest of the pre DLC bosses, although depending on your build, he might be a push over..