How do you one shot Holvik? assuming you hit for max damage (very unlkely) by your math you get 2d6+3+4+4 = 12 +3 + 4 +4 = 23 < 32 hp.
Yeah, so its not really a "one-shot" - you need to attack from stealth with main and offhand or with slash, surprise then attack once with mainhand, drink a potion of speed. This should leave you with one action, with Gehk yet to raise the corpses so then move east to cast web. Tbh I'm not very happy with this "run", since it relies on Gehk failing his save against the roaring thunder arrow which is pretty hit or miss.
Here's a log:
I'm not very happy with this build since it relies on bonus action for damage too much and armor of agathys is now useless in these higher level fights where things will one-shot you- so I have restarted with what I think is going to be a nice OP lvl 1 cleric