Sif is a mandatory boss that is required to complete the story, as he drops a ring that is needed in order to get into an area later on (you die without the ring on normally). Sif is a quick boss and he moves around a lot, so it may be tough to get hits on him. It may be better to use a slashing weapon on him, and he's vulnerable to fire and lightning. It's definitely helpful to have a shield with good physical defense on it. The biggest, and most dangerous, attack to watch out for is double sword spin move, which is usually telegraphed by when he turns his head towards the right side for about a second or two; you don't want to get hit by both hits, so either back away from him or roll towards him and hope neither hit connects. He also does a sword swipe, from left to right, and sometimes combos it with another in the other direction and then a sword slam to the ground in front of him. He also charges you and does a quick slash, and he can also jump back while swinging the sword. When the battle starts, most of the time Sif will use his charge move, which you can roll under if timed right. After this, though, the battle is pretty random as to what he'll do, so as long as you have a good shield with physical defense, you should be pretty good. A good strategy for melee users is to stick close to him and roll under his attacks to get under his belly, which allows you to get off one or two good attacks. For magic users, you will want to also have a good shield, but you will want to put some distance between you and Sif, getting spells off whenever Sif gives you the chance. Note that he starts limping once his health gets real low, which pretty much means you have won the battle, as his attacks will get a lot slower, to the point where they are pretty much easy to dodge with no trouble at all.