Fire Giant in True Oneshot with Giant Crusher | Elden Ring 

(Patch 1.09.1)
Edit: Added a detailed list of the buffs from my reddit comment.
Sure! These buffs are my most complex yet. To start, I use Seppuku twice to bring down my health. I then use a Frenzyflame Stone to build up madness, and sort my inventory by recent acquisition. I drink my Physick (Magic-Shrouding Cracked Tear and Spiked Cracked Tear), and call Torrent before the madness fully builds up. Then I jump off Torrent and use Howl of Shabriri at that precise location in order to fully inflict madness and increase damage with the Black Dumpling helm equipped. Once madness is inflicted, I swap the Black Dumpling to the White Mask and drink a Cerulean Flask. Then I dodge a few of Fire Giant's attacks to get close to him and use Soul Stifler so that he takes increased damage. After this I call Torrent and create some space from him, swapping the Winged Greathorn to the Commander's Standard while riding. When I get far enough away, I hop off Torrent and use Howl of Shabriri followed by Rallying Standard. Then I toss one Roped Fetid Pot to build up poison, and use Seppuku to inflict Blood Loss. Once Blood Loss is inflicted, I swap the White Mask to the Mushroom Crown and toss another Roped Fetid Pot to inflict poison. Once poison is inflicted, I swap the Lord of Blood's Exultation to the Red-Feathered Branchsword. I then call Torrent and try to get a little closer, hoping that he does the move that he did. Then I hop off Torrent, use Terra Magica, Royal Knight's Resolve on a Magic infused Giant Crusher, and then finally Contagious Fury with the Jellyfish Shield. Then I land a charged heavy attack, killing the Fire Giant in a single hit.
Edit 2: Added what was in the physick and adjusted a few minor details. Also the other talismans that I had equipped were the Kindred of Rot's Exultation, Axe Talisman, and Magic Scorpion Charm.
This is the second* true, glitchless oneshot of Fire Giant. There is one other person who has pulled this off with Carian Grandeur without using tumblebuff, and their name is ‪@AntoniaBelenBautista‬ . I am very impressed!