Titanite Slab from Siegward

You need one Titanite Slab to fully upgrade the Irithyll Straight Sword (or Irithyll Rapier) to +5. Once you have obtained the Hunter's Ring, you can properly one-hand the Irithyll Straight Sword and use it to fight these bosses: Oceiros, Champion Gundyr, Twin Princes, Slave Knight Gael, Soul of Cinder. These bosses, and some others, are weak against frostbite and the Irithyll Straight Sword is particularly effective against them. Complete Siegward's quest line and he would gift you a Titanite Slab.

Irithyll of the Boreal Valley

Location of Excrement-covered Ashes, Dark Souls III

Blood Gem at Irithyll of the Boreal Valley, Dark Souls III

Make your way to the Distant Manor bonfire and light it. Collect a Blood Gem on your way and use the Blood Gem to infuse the Bandit's Knife. The Bandit's Knife infused with a Blood Gem is an effective means of bleeding various bosses who are vulnerable to bleed. At SL1, a bleeding strategy is effective against the Demon Prince as well as Aldrich. Grab the Excrement-covered Ashes. Give this to the Shrine Handmaid and she would sell an unlimited amount of Dung Pies, among other new items. Dung Pies will be useful against a mini-boss at the Grand Archives. Find Siegward sitting in the kitchen of Irithyll. Exhaust his dialogue to trigger him to relocate to Irithyll Dungeon.

Irithyll Dungeon & Profaned Capital

Estus Shard at Irithyll Dungeon, Dark Souls III

Titanite Chunk x 2 at Irithyll Dungeon, Dark Souls III

Location of Old Cell Key, Dark Souls III

Location of Xanthous Ashes, Dark Souls III

Grab the Estus Shard found inside a Mimic above the Giant Slave. You should now have 13 uses of the Estus Flask. The Giant Slave drops one Titanite Chunk. A nearby Crystal Lizard drops another Chunk. Collect the Profaned Coal from inside a cell containing a bunch of Wretches. Outside the cell are several Jailers circling a courtyard. Give the Coal to Andre and he would be able to infuse your weapons with Blood, Dark, or Hollow. You need the Blood infusion for the Bandit's Knife. An adjacent cell contains the Xanthous Ashes. Give this to the Shrine Handmaid and she would sell an unlimited amount of Gold Pine Resin/Bundle. The Old Cell Key is found inside a chest, opposite which is a Mimic. Opening the chest would trigger a gang of Basilisks to drop down from above. The key is required for unlocking the jail cell where Siegward is held.

SL1 How to get Logan's Scroll, Dark Souls III

Undead Bone Shard at Profaned Capital, Dark Souls III

Titanite Slab from Siegward & Covetous Gold Serpent Ring, Dark Souls III

Location of Prisoner Chief's Ashes, Dark Souls III

Location of Jailer's Key Ring, Dark Souls III

Light the Profaned Capital bonfire. An Undead Bone Shard is located right next to the bonfire. Your Estus Flask should now be +9. Defeat the crystal sorcerer and grab Logan's Scroll. Give the scroll to Orbeck to advance his quest line. Rescue Siegward, exhaust his dialogue, and be rewarded with a Titanite Slab. Near Siegward is a corpse containing the Covetous Gold Serpent Ring. The ring is useful for farming rare weapons and items. Consider using the ring, together with the Symbol of Avarice and Rusted Coins, to farm the Lothric Knights at Lothric Castle for Titanite Chunks to upgrade various secondary weapons such as the poison infused Rotten Ghru Dagger. Grab the Jailer's Key Ring from a nearby room. You need this to unlock a cell containing the Prisoner Chief's Ashes. Collect the Prisoner Chief's Ashes and give the item to the Shrine Handmaid and she would sell an unlimited amount of Pale Pine Resin. The consumable applies magic to your weapon and is effective against the Dragonslayer Armour.

Yhorm the Giant

SL1 Yhorm the Giant, Dark Souls III

Enter the boss arena and dash to the back of the room to grab the Storm Ruler. Since you have rescued Siegward, he would also join in the fight against Yhorm. Use the weapon art of the Storm Ruler to quickly kill Yhorm.