The easiest way to take on the Gaping Dragon is to try and stick close to the right side of his body, between his two legs. This location allows you to easily dodge his side swipes, and gives you ample time to avoid his stomps. When he flies into the air, move away as to not get crushed. Be prepared to turn and run, as he occasionally vomits a large pool of acid.
A Lord Demon encounter, Nito is pretty brutal. First of all, the dude's entire body and his sword are made of skeletons. Secondly, one of his main abilities is to spew death from his mouth. Within his chamber are several skeletons, some of which are the big brutes that knocked us off ledges in the Tomb of Giants a billion times. So, when you arrive in Nito's chamber, lure him toward the entrance. Try rolling around until he reaches you, as he can use a magic sword attack from anywhere in his room that pops up through the floor. Luring him over should let you avoid having to deal with the giant skeletons, though you'll still have to put up with a few of the small fries.
Nito himself is fairly easy. He has a few standard sword attacks that hit pretty hard, but aren't difficult to avoid. The problem is that while you're trying to damage him, you'll be taking hits from the skeletons nearby. Oh, and you can get poisoned during the encounter. Get a few hits on Nito. Eventually he'll spray death all over, temporarily destroying the skeletons. Quickly get some more hits in before they respawn. Just heal up whenever the opportunity arises, and he'll be dead in no time.
Great Grey Wolf Sif
Ah, yes. One of our favorite fights in the game. Sif (who somewhat resembles Okami's Amaterasu) is kind of a dick. His attacks are strong and are sometimes tricky to dodge. You'll want to play it safe. Wait till he finishes a set of attacks, sprint under his body, get a few attacks in, and get out.
You'll especially want to look out for his two-part spin attack. Even if you block the first swing in the attack, it will drain most of your stamina. The second strike is sure to knock out at least half of your health. You can tell he’s about to do this when he switches his grip on the sword in his mouth, even if you can’t avoid it entirely, taking one hit is way better than taking two. This fight is all about patience. Once Sif is below 25% health you’ll notice he starts limping, there’s a way to end the fight without killing him.
Here it is—the final battle. Take a moment to appreciate the piano music in the background. Everything you've done has led up to this point. Gwyn is tough. He attacks fast. He hits hard. You'll have to roll to avoid many of his attacks. We found the easiest way to approach him is to constantly be walking backwards with your shield raised. Equip any stamina regeneration gear you might have, because you'll be rolling and blocking a lot. Whenever Gwyn tries to kick or grab you, use that opportunity to get a few hits in, then start backing away. Don't get greedy—trying to land too many hits too soon can easily result in death. When you need to heal up, try and line of sight him around one of the pillars in the cavern.
The Iron Golem isn't all that hard. Note that you can fall off of the platform you're fighting on, so try to keep the boss in the center of the tower. Circle strafe to avoid most of his attacks, and back away when he tries to pick you up. Also, be wary of his stomp attacks. Note that even if you block his attacks, you’ll slide back quite a ways, so make damn sure you’re not blocking near the edge of the platform. After he takes a bit of damage, he'll be stunned for a substantial amount of time. That’s when you casually end his life.