Veteran
Hitting at least 3 enemies with an attack, increases your damage by 24-36% for 3 seconds.
Hitting at least 3 enemies with an attack, increases your cleave by 140-200% for 2 seconds.
10% Critical Chance for every 0.2-0.35 second while aiming. Stacks 10 times. Discharges all stacks upon firing.
14-20% Strength on Salvo's First shot.
For Killshot Ready - 2500 kills will take a while. Take Camouflage to gain stacks while standing still or crouching and dodging. Sprint between encounters, but once you're in a place for some time - pick a place, crouch and shoot everything. Crouching is mostly better because you can dodge as needed. Meleeing while crouched sucks imo as it's hard to headshot. As long as you have a wall behind you and it's just poxwalkers - I found you can stand up and just swing at your enemies without dodging to keep the stacks. Take something with decent cleave and swing speed like a Chainsword so you can keep all of those poxwalkers staggered while doing this. Once you're getting pressured by bruisers and ragers - better abandon the stacks for a bit and just fight normally.
Marksman's Focus adds finesse bonus, that is a bonus to crit and headshot damage, so outside of focusing headshot - weapons with high crit multiplier will be a bit more powerful.
I did Killshot Ready with a Vraks 7 Headhunter, with Surgical and Opening Salvo. Shooting a bit slower so that the salvo activates makes sure that 1. your shots pierce more enemies and 2. you get some surgical stacks for more crits, which are especially potent with 10 marksman's stacks.
You can also simply take an IAG with more crits like 5% crit talent, 5% crit perk and Deadshot - they'll crit fairly often and extra powerful. Agripinaa did best for me here because I was shooting from afar often.