If the Chosen Undead saves Sif in the Chasm of the Abyss before killing Sif in Darkroot Garden, a different cutscene will play before the fight; however, this can only be done if the player places the Lordvessel in the altar through Firelink Shrine, as Artorias' ring is needed to access Kaathe. In the cutscene, Sif will recognize the player as they approach and let out a whimper. The protagonist reaches out to pet the wolf, but Sif pulls away and howls mournfully before sadly picking up Artorias' greatsword. The fight will continue as normal after this point.
For melee fighters, getting underneath Sif is generally the best strategy, as it makes most attacks miss the player character completely. Care must still be taken to dodge, block or move out of the way of the slam attack, though. Locking on is not always a good idea when doing this, as it can make the camera spin and make it harder to control one's position. Two-handing is more effective, as it will make the fight shorter and reduce the number of slams that need to be dodged.
For ranged characters, players can simply target-lock Sif and kite Sif backwards around the edge of the arena. Staying as far as possible away from Sif will cause most, if not all, of Sif's attacks to miss.
Using miracles, sorcery, or pyromancy to soften Sif up before moving in with melee attacks can also prove to be effective.
Using a summoned phantom as a decoy can make positioning oneself under Sif easier. In a party of two or more players only one should try to get under Sif, otherwise players may bump into each other when trying to dodge Sif's attacks.
Sif's attacks can be blocked, but most of them will cause a significant loss of stamina, even with a good shield. Learning Sif's attack patterns and avoiding them completely, either by dodging or simply walking out of the way, is highly recommended.
Before the player enters through the fog gate, there is a broken piece of bridge to the left that can be jumped onto. This is most easily accomplished by unequipping all gear beforehand. After reaching the bridge there is another pile of rock resting at the top that can easily be reached through jumping. When at the top, Sif will be visible. Sif will be unable to reach the player, but the player will be able to damage Sif with arrows or other ranged attacks.
Sif will start visibly limping at critical health, which vastly decreases the wolf's speed and agility. Sif's moveset also changes, having less attacks and the ones remaining being much slower and having much more ending lag.
Sif also limps at critical health when summoned to assist the player with Manus.
It may be hard to summon a phantom before the battle, as Darkroot Garden is a PvP hotspot, allowing invasions.
Sif repositions the sword to a straight position, and spins around twice. Causes massive stamina damage if blocked and can kill players if the slices connect. However, if Sif is at low enough health to where he is limping, he will only spin the sword once. Can be dodged by backing or rolling away before the attack is unleashed. Sif will pause afterwards, creating an opportunity for attack.
Sif will attempt to slice the player while jumping backwards and away from the player in one motion. Has little to no starting lag. Blocking it won't give the player an opportunity to attack, but will help the player survive.
Sif will do a quick chop from side to side. Good tracking and high hitbox. Possible to roll underneath it with good timing. Also does not do much damage to the stamina bar, thus it can be blocked quite easily.
Two slashes from side to side. Great tracking, thus very difficult to sidestep. However, the slices can be rolled under and blocked fairly comfortably without the stamina bar being fully drained. If Sif is at the point where he is limping, he will fall on his side and struggle to get up on attempting the second slice, so use this to your advantage to do some damage.
Sif is a traditionally female name in Nordic society (specifically, it's the name of a goddess of Earth), however Sif's actual sex is never clearly stated with in the game.
Sif comes from the Old Norse word sifjar, which translates to "affinity or connection by marriage". This may be symbolic of Sif's strong relationship with Knight Artorias (and perhaps the Chosen Undead if the expansion DLC is taken into account).
In the fight against the Chosen Undead, Sif wields the sword of Artorias, former companion who sacrificed himself in order to protect Sif.