Using the DLC attack gift " Steadfast Strike" with any decent damage 2-handed sword can net you 99k damage. This build is the same as the halberd 1-shot build in previous videos with these changes:
Use 2-hand sword mastery, use steadfast strike.
You still need to swap one passive (mind/ str up) to survival instinct, and you will still have to swap out blood sacrifice for steadfast strike once you use it for the last time to grant yourself enough Ichor to be able to use steadfast strike.
And thats it. Timing these 1-shot builds on the DLC bosses is a bit tougher, as they all have different points of weakness, and their aggression means they move around a lot more. If you do not hit their weakpoint, 99k will more likely end up being half or less the damage, so it takes some practice to actually pull it off. Even if you only land for 70k, that still significantly decreases the difficulty of the DLC bosses.