The only other main boss that might be challenging is a group of 3 beings. Otherwise, they're all pretty easy (the main story bosses). Don't forget to parry with the gun. It's really helpful. Annoying, but helpful

The required bosses aren't terrible. Shadow of Yharnam can be tough, Vicar Amelia is a pain in the ass, and Gehrman can be a nightmare. That said, if you want challenges, make sure to do the optional bosses as well.

Then, if you want true hell, the DLC ramps up the difficulty profoundly.

The bosses are pretty easy. I beat all of them at least on my second try except ebrietas daughter of the cosmo that took me a while to beat but the good thing is that she is optional. The hard part is to get to the bosses.

I killed most bosses on my 2nd or 3rd try, but I died at least 20 times against Gascoigne. I bet you'll face your personal archenemy soon enough haha

Bosses necessary to advance the game:

Father Gascoigne
Vicar Amelia
Shadow of Yharnam
Rom, the Vacuous Spider
The One Reborn
Micolash, Host of the Nightmare
Mergo's Wetnurse

Optional Bosses:

Cleric Beast
Bloodstarved Beast
Witch of Hemwick
Darkbeast Paarl
Amygdala
Celestial Emissary
Ebrietas, Daughter of the Cosmos
Martyr Logarius
Gehrman, the First Hunter
Moon Presence
Ludwig the Holy Blade (DLC)
Laurence the First Vicar (DLC)
Living Failures (DLC)
Lady Maria of the Astral Clocktower (DLC)
Orphan of Kos (DLC)
Abhorrent Beast (Chalice)
Beast-Possessed Soul (Chalice)
Bloodletting Beast (Chalice)
Keeper of the Old Lords (Chalice)
Loran Darkbeast (Chalice)
Loran Silverbeast (Chalice)
Merciless Watcher (Chalice)
Pthumerian Descendant (Chalice)
Undead Giant (Chalice)
Watchdog of the Old Lords (Chalice)
Yharnam, Pthumerian Queen (Chalice)

Gehrman is NOT mandatory for all endings, only two of three. And as you can see, the number of mandatory bosses is actually fairly small. In addition, a number of bosses are repeated in the Chalice Dungeons. These sometimes use alternate versions (Defiled Amygdala, AKA the real Hunter's Nightmare; Headless Bloodletting Beast; etc.) This also doesn't include the optional Hunter Encounters, such as Eileen or Henryk.

Get the DLC if you allready like it. You can play it early in the game and get all kinds of extra weapons and items from the DLC which will help you a lot for the main game.

I too just started playing it. It's brilliant in every way except the grinding. Mother fucking GRINDING is the bane of my entire soul. I hate it and I have no idea why anyone, anywhere, would want to do the same thing over and over and over again... ugh.

Somehow I found Father Gascoigne was one of the hardest boss for me, but only because I didn't know how to play and move properly, it was a frustrating boss that's sure.

Indeed I beat him without knowing that I could dodge back.

yeah I figure that out early on but for some reasons, after being close to killing her many times, she found the way to kill me due to lack of concentration.  Still, she was the only one that made me sweat.

Why is everyone telling him it gets harder? I genuinely found the first few bosses (Gascoine, and Blood Starved Beast being the two major culprits) were the hardest. Mostly because I was getting used to the controls, the way the game played, and other stuff.

Don't get me wrong, shit was hard all the way through but the beginning was hardest because I never beat a Souls game before.

As for the story, it's mostly relegated to the background. It's all there (and it's insanely deep), but you could play the entire game without catching onto about 95% of the story. it's all incredibly subtle storytelling.