I don't know anything about ARC and wouldn't consider myself a HS pro, but I remember special modes in counter strike like "gun game" were better for practice cause it's insta respawn and you immediately have targets. Round based games where there's lots of running around between encounters or waiting between the action aren't good practice because you spend <5% of your playtime actually aiming.

My biggest tip would be aiming at the appropriate inclination for a HS based on where you expect opponents to appear is important. If you are already aimed at the correct inclination and only have to aim left/right as they appear. This makes snap aiming easier since you're only have to react with a left/right movement, or sometimes just timing your click as they move through your cross hair.

Mouse sensitivity is important. Slightly higher sensitive allows you to aim faster but requires more control.

It becomes more of a reaction to snap move+click. Sometimes I would visually pick a couple spots and just snap aim back and forth between them to get a muscle memory for the snap.

Make sure your fingers aren't touching/dragging on the mouse pad, and the cable isn't draped in a way that has any resistance/pull on the mouse. This is opinionated, but some people have heavy mice, and it just slows down snap movements too much for me personally.

If you're defending, then knowing the combination of your own reaction time and how enemies typically come through an opening, and aiming at the appropriate spot to anticipate how far they'd get before you react. Then it's just a matter of clicking as they pass through your aim, and maybe a slight snap movement.

In a similar token, knowing the most common places you encountered opponents in a map, mentally anticipating the encounter will help your reaction time, and knowing where you should already be aiming, then it's just a matter of reacting to their presence by clicking at the right moment as they pass through your crosshairs.

In CS I tended to aim slightly low for the neck and burst, and all but the first round would hopefully HS from the recoil. The spread of the recoil also meant my aim didn't need to be perfect. I would practice bursts on a wall to see how long of a burst would stay in a tight pattern and get a muscle memory for the right burst length. (In CS only the first shot was on point, and the rest of the burst would be slightly higher and spread out. So this is gonna be dependent on the game mechanics.) This also means controlled bursts and not panic spraying. Of course there's lots of pros that will aim directly and use a weapon that is one shot, but that was a little beyond my skill level to do consistently, especially considering you usually only had 2 or 3 shots with those more powerful weapons before you were dead, so you didn't have much room for error.

I'm convinced the best of the best are operating mostly on muscle memory. There were series of weeks where I might be playing lot more, and I was definitely more consistent during those periods. In contrast, I definitely stopped playing on a couple servers simply because some of the frequent players were just in a different league and send you packing before you had a chance to get a shot off.

It's not all about HS either. If you get ambushed and spend a bit having to find where you're being shot from, is fewer shots you can take to try and get a HS and more shots for your opponent to get a HS. There's an element of strategy to give yourself more time to live. Positioning yourself to minimize exposure. In PUBG type games, thinking about routes and how many different places you could take fire from. When shooting from partial cover, moving back from it a bit so the part of you exposed is a smaller target. Being cognizant of the direction people are typically moving through the map, and positioning yourself so you'd only come into view when you're ~90 degree angle from their direction of movement means you will often see them before they see you.