I don't know. I thought I was supposed to be able to take him down. I killed 1 boss in Heide, cleared all No Man's Wharf up until the boss in the ship which I haven't attempted yet. I think I just suck, I randomly die to any mob when I start up the game and it takes a few deaths to get used to it again. Ive got the Knight armor +3, Fire Longsword +3, leather shield, 4 rings, estus 4 +1. I just utterly fail at fighting. I feel bad leaving so many bosses behind.
No it's not you sucking. Even the lowest mob can kill experienced players if they slack. It's how it's supposed to be!
In general, my advice is drop the shield, try two handing a spear, pump agility and health. Shields cripple your development and aren't much fun to use. I feel kinda silly rolling everywhere but the game seems to be built around rolling, so you'll have the most fun by doing it.
The spear is my favourite early weapon if you two hand it. It might seem basic at first but that's good if you're new because it allows you to focus on the enemies and your positioning instead of worrying about sexy combos. Plus it does the basics really well. Thrusts are OP against AI mobs, especially if two handed because it will stun them. Give it a try with your longsword and you'll see how great they are. But the swing is also really good. Combined they give you the two best types of damage (thrust and blunt) plus the options of forward or wide attacks so you have an attack for all occasions. Also, if you use the range wisely it can help keep you out of harm so you shouldn't miss the shield too much. Plus they have the best backstabs EVAH!
As has been said, get your agility up. Not being able to do things isn't a lot of fun. Don't worry about damage too much because staying alive and being able to do things is more important. Get the stats you need to use a weapon then pump health along with some stamina. High health makes the game much more enjoyable. Low health should be for when you're much more experienced IMO.
Don't forget backstepping, dashing and jumping attacks. They can make combat a lot easier and a lot more fun. If you haven't killed the Old Dragonslayer yet, try backstepping his initial attack. It's really satisfying to pull off.
For bosses, my advice is put your soapstone down and get summoned a few times before you summon anyone else. It's a great way to get practise without having to keep fighting your way back to the boss over and over. Plus, if you're successful you get all your shit repaired/refilled.
I'm not a great DS player and I haven't touched the game in like 6 months. But I'm getting an itch to play again so if you want to do some coop let me know. Sunbroing is my favourite part of the game so I'd love to help out. The lack of coop is actually why I quit playing. There's a lot of cool stuff in the game you'll never experience if you quit. Apart from later levels there's also NG+ which is very different. Don't worry about being badass or feeling like you should solo everything. You paid good money, so have fun and get your moneys worth!