Qada's not even that hard. And it's not even worth increasing def/stats by leveling up because his attacks are item-based and will ignore defense. Poison Immunity would help so equip that. If you actually watch what he does, he has an attack pattern.
Your biggest mistake was using Damage Dispersion. You want someone to cast dispel to nullify his regen. Don't hit him until he uses 1 Dark Breath attack because he usually combos it with a poisoned hi-potion. The next one (next round) he tries to fire off something but doesn't work so you can take the opportunity to hit him. I hope you're not Braving too much because that's silly. At the same time, don't bother Defaulting because as I said, he hits with items and that ignores defense. Additionally, if you use Ninja, DB counts as a physical attack and can be countered with Utsusemi, so if you wanted to wait until the end of chapter 3 to beat him instead and get Ninja, might as well do that.
iirc, if he defaults, he'll do the DB + Poison combo so just attack anyway because he'll kill whomever he targets, and use the round after (the free one) revive/recover, and get a hit in.
Corvo, man, the game's not that hard. Learn to use your abilities and look at enemy attack patterns and you'll be fine.