Lol that was me playing.
I've forgotten what level I was going into this fight.Maybe 70?I was 76 prior to fighting Successor of the Throat, so I was at least that.
As for tanking them, that was pretty much my idea going into it. Increasing attack buffs and using Cleansing Light for healing half the damage dealt back to me while trying to take down Blade Bearer.
Fire tonic and ice tonic were used pre-fight to help with the tanking.
Damage-wise, that's probably based on Gifts. Noble Silver buffs Light gifts, which in this case meant stacking Final Journey, Bridge of Glory and Adrenaline. Cleansing Light also counts as a light gift. Throwing that into a Fortified Zweihander gives pretty good results.
As time went on throughout the game, I've found that my success corresponds quite well to how many successful Dragon Lunges I end up landing on a boss.