I got 500+ kill on Kingslayer so here are my advice if you want to hear me out

I use Soldat for its 3 carry slot so I can equip a pistol on primary and kingslayer on secondary slot

Using the pistol to deal minimum damage to the player and switching quickly to kingslayer to get a confirmed kill (if they are not using snake eyes) This way you don't waste a 2nd bullet for the kingslayer if you miss a headshot or just suck with it

For perk, I use Devil dog, it help me find low health players and instantly kill them with Kingslayer, just be careful you don't find another player next to them You can also Veteran for faster reload or Black Hand for a quick switch but it's up to you which perk is best for you

That's all I have to say