[quote=Jorick]
I just started playing last week, no online stuff, and I've been able to handle everything just fine. There are NPC guys you can summon for help on a lot of bosses, and they've been plenty of help for the... actually, just one boss that I couldn't tackle solo (area boss for Anor Londo, and I almost got it on my one solo attempt after a couple assisted ones, then summoned the guy and wrecked it hard that time), the other ones that I summoned NPCs for I ended up dying on those attempts and ended up beating them by myself.It's really just a matter of learning attack patterns, playing cautiously, and picking your gear for the encounter rather than just sticking to one set for everything.
[/quote]

The one case I always think of when talking about the use of co-op is Ornstein and Smough. Probably the most infamous boss battle I'm aware of, to the net, and it's the most common boss fight I see get people to ask for fellow players to get on and help with in a forum.

Going solo's not impossible, by any means, it can just be a giant pain the ass.