I found the beast to be the toughest, but only when choosing to protect abigail (with the villagers it's a piece of cake because you can drag the beast out of the barghest spawning zone then). It does pretty hefty damage for such a low level and the extra barghests spawning will eat away additional hitpoints fast enough for your swallow potion not to keep you alive, so you have to run around a lot and try to stay away from their fangs and wait for abigail to get up again in order to regenerate health, particularly when you get stunned by the beast that's pretty damn hard and at that level you don't have very high defenses yet.I killed nearly all the other bosses on my first attempt (all except Javed). The first time I killed the golem didn't even do it with the lightning pillars, because I thought I had to pull the guy to the tower itself (according to some information I got about the thing, I think from kalkstein). I just fought it using swallow and that potion that slows everything down and increases your defenses by a lot, and when that potion ran out, I just ran around and blasted the thing with igni. It's slow anyway. He is.. too easy
The Koschey I didn't have many problems with.. just used Swallow and that was just enough to keep me alive through the fight.The only other fight I did have some problems with was Azar Javed. The damage he does seems somewhat random, so sometimes he just totally wrecked me, but after a couple of tries I got one attempt where he didn't do much at all and then I nailed him. I did use all the tricks I had up my sleeve to finish him quickly by the way.. Swallow, Tawny Owl, that potion to reduce knockdown, rubedo, albedo, nigredo and and the strongest rune on my weapon.Funnily enough, most of the times I got in trouble during fights wasn't while fighting the bosses at all. Large groups of 'weak' enemies were usually a lot harder, even in group style, and I did die frequently in those.The reason I found the beast harder than Javed is because at the beast I didn't have all the 'tools' available. Otherwise he would've been easy too.