The spider was awful too, I remember. Though I didn't struggle all too much with Allant, really. I think got him like 3rd try or so. He's the only boss in the game that felt like a "classic" standard souls boss and not like a gimmick, so I got into it rather easily. Flamelurker was a pain, the spider was a pain. Outside those two, I don't remember any other boss being that much of a roadblock:

Penetrator was also pretty easy but a cool fight overall.

Fool's Idol was a bit of a challenge, I think, because I didn't get the gimmick immediately. But I did kill that sorcerer up top that buffed her (was it even that fight?)

Adjudicator was just annoying and killed more often that I wanted it to because I didn't get the gimmick here either. In hindsight, it's obvious what you have to do, but if I've never used a bow before, how would I think of doing so here?

Oh yea, Maiden Astraea was also annoying because her anointed knight was more of a annoy-ntet (heh) knight. Got parried all the time and they dealt way too much damage.

All the other bosses I've not mentioned were fine overall.

Maybe I should go back for another run sometime?