As the toughest boss fight in the Ashes of Ariandel DLC in Dark Souls 3, Sister Friede has become a fan favorite for a ton of reasons; including cool design, fascinating lore, great references, and providing a challenging battle. Fighting her, and Father Ariandel, through 3 brutal phases will truly satiate any Ashen One's thirst for ember-filled combat.
Though, there are many who (understandably) struggle with this boss and need to rely on summoning allies to defeat the duo of Sister and Father. Friede's incredible speed, invisibility, and power make her a force to be reckoned with, however, there are some solo ways to overcome this scythe-wielding maiden of ice and black fire.
Melee Stun Strategy - Stagger Friede With A Strike Weapon
Of all the damage types in the game, Sister Friede is only weak to 1 of them: strike damage. As such, the weapon types that will be able to remove chunks of her health bar the fastest are the hammer and great hammers. These kinds of armaments also deal more poise damage on average than most other weapons, which is also very helpful against the light-footed lady of the Painted World, for she has very little poise and can be staggered relatively easily with the right weapon.
The great hammers will essentially grant the player the ability to stun-lock Sister Friede until one's stamina bar runs dry, however, the faster attack speed of the one-handed hammers may be more advantageous for being able to unleash more strikes in a quicker manner despite not being able to stagger her quite as much.
If using this tactic, be sure to focus on the different aspects for the 3 different phases. During phase 1, be aggressive, though do not let her land a critical invisible attack by running back and forth in the center of the large room until she reappears to throw off her targeting. During phase 2, target Father Ariandel, as he is easier to damage and the shared health bar between him and the swift Sister make this phase not too difficult as long as one keeps an eye out for Friede's attacks. During phase 3, be patient and wait for an opening, as getting caught by one of the dual-scythe-wielding damsel's devastating attacks will put the player in the utmost distress.
Melee Bleed Strategy - Use A Bleed Weapon & Carthus Rouge
Although Sister Friede is only truly weak to strike damage, there are other types of hurt that one can inflict upon her and Father Ariandel, particularly the bleed status ailment. With the right weapon and plan, the Ashen One should be able to fully fill the bleed meter of each foe in this fight after only 4 or 5 hits. The ideal way to do this is to use a weapon that has innate bleed build-up, such as one of the fan-favorite katanas or the immensely versatile curved swords.
Grab one's preferred bleed weapon (make sure it can be buffed before infusing and upgrading it) and also buy a bunch of Carthus Rouge from the Shrine Handmaiden (sold after giving her the Gravewarden's Ashes). Stacking the natural auxiliary effect of the weapon with Carthus Rouge should allow one to remove large portions of Friede's health bar via bleed build-up, though will be even more deadly against Father Ariandel, as he has a great susceptibility to bleeding, which is canonically appropriate due to his constant self-flagellation. Be sure to use the melee tips for each phase that was mentioned before, as they will also be useful for this tactic.
Spellcaster Strategy - Use Pyromancies, Miracles, & Dark Spells
Despite Sister Friede using pyromancies, miracles, and dark magic spells throughout the fight's 3 phases, she is not immune or resistant to such attacks from the player. The only things that one should never try to use against this boss are magic damage, for she resists this, and frostbite, which she is immune to.
Regardless of what spells are used, players should try the ones that are faster to cast and have faster projectile speed, as Friede is very agile and will try to evade many of them. During phase 1, do loops around the center of the room to kite her around the place while also keeping enough distance from her to cast a spell here and there. This entire fight will be about endurance and patience, so be sure to only attack when there's an opening, as getting hit while in spellcaster robes can be incredibly dangerous due to Friede's high damage output.
During phase 2, like with other strategies, target Father Ariandel because of his many weaknesses and big hitbox while also keeping an eye out for Friede's attacks. It should not take long to drop the larger figure, especially if one uses Pestilent Mist on him and simultaneously rains down other spells while the haze eats away at the shared health bar.
When phase 3 is reached, be extra careful, for this phase has the most dangerous attacks, however, these massive maneuvers by the frigid Friede will all leave her noticeably vulnerable after each one. Bait out her big attacks by staying away from her (while also staying within lock-on range) and unloads a barrage of fast spells between her assaults. Even though this battle will be very challenging for spellcasters, these tips should help one stand up to Sister Friede's fierce fighting style.