IIRC, they don't negate 100% of the damage, rather that it only means the KQ is highly resistant against that type of attack. So I want to solo KQ on a slayer task, is it better to use melee + range on her or just stick with melee the entire time? What's an optimal setup and stats for this? 

 

Also it is better than using Veracs? If not, I'll probably not consider it.

Melee and Range if maxed range if not Veracs.  You can pure melee with Veracs since it has the prayer ignore.  Veracs is a much simpler option than bringing range but it will slow down the second phase compared to 99 range.

I'm not sure exactly how much the prayers negate damage, but you're right that it's not 100%. I think it's just something like "75% not to hit through prayer" BEFORE the actual accuracy calculation takes place, either way, it makes  typical melee not a good option when it's protecting from it.

 

With your levels I think you should use whip and trident on the correct phases.

 

Due to the nature of KQ's attacking I'd recommend duo even at max combat, it's just not worth it to solo.

I think it's the same as protection prayers in pvp, except she can use more than one at a time

KQ sounds a lot tougher than I imagined. Only time I ever solo'd her on the other RS was with overloads so I guess I underestimated her.