Help with a ST skill for my build

First off: thanks for clicking on my thread, hopefully you have the time and the insight to provide me with the help i need. Thanks.

My guy is doing well at clearing packs. He is clearing them very fast, doing Merci Dried Lake is a breeze. FrostBlade CoC with Frostbolt is insane for clearing large groups.

However, Single Target (ST) boss damage is abysmal.

I have a 4L free in my gear to slot suitable ST skill gems into.

Build details below:

Tree


Skills


Gear


I have tried several ST combinations that did not work to my satisfaction:
Double Strike + CoC + Multistrike + (Bladefall or Frostbolt or Freezing Pulse).
Viperstrike + Rapid Decay + Void Manipulation + Faster Attacks (poison stacking)

Thanks for reading, please let me know what a decent 4L set up is that will at least let me take Voll down quickly in Merci.

Both of your weapons have only fast attack speed and high crit chance. This makes them really good for cast on crit, but really terrible for anything else since they have almost no base damage. No skill that scales off physical damage will be good for you (viper strike, double strike) nor will scaling anything other than cold be good for your build since that's what you chose to invest in in your tree. This seems like a downfall of your particular build, there aren't really any efficient single target setups for you. I would personally try weapon swapping into a bow and using barrage to cast on crit for single targets, there really aren't any dagger skills that will get you a higher proc rate, and you can't efficiently scale the damage of anything other than cold spells with your build.