Killshot is an amazing facet. It turns a boring ranged nuke into the ultimate skillshot.

The way it works is that when it hits a hero, it applies its damage, and then checks to see if they are below the HP threshold for execution. The minimum is 5% for a Powershot that was fired instantly, and for a fully charged Powershot, the threshold is 9%/10%/11%/12%. If the current HP after applying the Powershot damage is below the threshold, they execute. It is effectively a ranged Culling Blade.

So for a 1,000 HP hero, the execute threshold is 120 damage. Assuming they have about 35% magic resistance, in order for a lv 4 Powershot to execute them, the 470 magic damage needs to get them to 120 HP or less.

x = 120 HP + (470 magic damage x (1 - 0.35))
x = 120 HP + (470 x 0.65)
x = 120 HP + 305.5
x = 425.5 HP

If you hit somebody at about 425 HP with the Powershot, they die. And the threshold only goes up over time.

A hero with 1,500 max HP has an execute threshold of 180 HP + 305 = 485 HP.
A hero with 2,000 max HP has an execute threshold of 240 HP + 305 = 545 HP.
A hero with 2,500 max HP has an execute threshold of 300 HP + 305 = 605 HP.
A hero with 3,000 max HP has an execute threshold of 360 HP + 305 = 665 HP.

So as long as you do not miss, you can kill people who are at 400-ish HP out of 1,000 max HP at the end of laning when you are level 7. Every shot gets the enemy heroes closer to death, and they should be scared for their lives. All you have to do is not miss, and also have enough mana to fire that snipe.
Lategame, you can kill any enemy hero who is hovering around 2 bars of HP.

And the best part is that the execute will ignore things like barriers (Glimmer Cape, Medusa Mana Shield, etc.), untargetable enemies (Escape Act, Shadow Realm, etc.), and BKB and similar (because they only give 60%-80% magic resistance, not full magic immunity or debuff immunity. So a 1 HP Juggernaut spinning away can be execute).
The barriers will block the damage from the Powershot but if the target hero is low HP and below the threshold, they will still die. So when you see a low HP enemy get away with shields and such, do not give up! Keep sniping!

There are some things that block the execute. This is not a complete list but the following do not work from my experience:
- Aeon Disk
- Wraith King wraith form
- Invulnerable targets like enemies in Omnislash / Ball Lightning / Tornado
- Borrowed Time from Abaddon (it used to, not anymore)
- False Promise from Oracle (it used to, not anymore)
- Shallow Grave from Dazzle
- Troll Warlord ult

This guide does not do anything special for Powershot specifically, but this is how I play core WR in games, and also this build is how I farmed ult damage for the arcana so I figured I would just put all of this into one guide for others to use when they play core WR or try to farm arcana damage. Also I wanted to put in an in-game table in this text box so you can check on it in-game.

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Tips for arcana farmers:

If you want to farm that arcana, play WR aggressively (even as a support), and get lots of kills. Yes, Killshot will make it harder to get ult kills because of the execute but that is okay. You can only kill one person at a time until you hit lv 25 anyway, so might as well use those Powershot kills for fast farm and levels. Thin the herd with Killshots, and then chase stragglers down with the ult to secure the damage. Just remember to not feed. If you die before the target does, the damage will not count. You need to be alive, and have the Focus Fire buff active on you when the target dies for it to count.

Average arcana damage in a 30 min game: +10k
Average arcana damage in a 60 min game: +25k - +50k

Took me 3-4 months over ~300 regular games (not turbo. Turbo gets -50%) to hit 2.5 million damage. I think that is a good ballpark of how long it will take other people too. It really helps that you do not have to win a game to keep the damage. Yes, you keep the damage even if you lose. So just play the best you can every game without throwing/stalling/holding the ult and you'll get there.

Happy hunting!!