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"So, first thing to remember is that it’s based on killing, not necessarily killing with your warp abilities. I know that sounds obvious, but remembering that is what helped me through it.
Get a revolver with high power that can one-tap most special enemies, and you’re off to a good start. It’s really good for picking off enemies at a distance, or singling out an enemy in a horde. This is a good way of gaining warp charges, and also a good “oh Emperor” button for when your charges are threatening to fall off; find the nearest special, pop them in the face, and you’re back in business.
Next thing, you’ll want to play on a shock trooper gauntlet. If you can handle a high-intensity shock trooper gauntlet, even better, because more basic enemies means a higher chance to trigger that 4% chance. A map like the deep archives is best; the narrow corridors and quick sequence of encounters as you descend means a lot of specials back to back, and hordes crammed into the corridors means it’ll be less likely to run into a situation where you need something to kill but the nearest special is three miles away.
Assail is your best option; it deletes hordes, and just about anything else you throw it at. That means more kills, more procs, more charges, and less chance of your allies stealing your kill at the worst possible second.
Lastly, and this is probably the hardest part, you need to pace yourself.
The TL;DR is that once you hit maximum stacks, you want to wait as long as you reasonably can between special kills to make sure you get as much time off of each individual kill as possible.
As for the “longer words explaining,” as Melk would put it…
It takes 4 kills to reach max stacks, and that’s just to start the timer ticking. You keep a stack for 25 seconds, and you refresh that 25 second duration each time you kill something.
Why is this important? Because, hypothetically, if you could kill something at exactly 25 seconds and refresh it without it counting as having fallen off, it would take a total of 16 kills to hit the full 300 second requirements (300 / 25 = 12. 12 + 4 = 16).
Meanwhile, if you killed something every 10 seconds, on the other hand, it would take 34 kills (300 / 10 = 30. 30 + 4 = 34). Well, 32, technically, because once you hit that point, your 25 seconds from that most recent kill would carry you across the threshold, but the point stands.
In short, you need to hold off on killing specials immediately, as long as you can do so without dying or getting the group killed (or worse, risking the group killing them before you do). If you kill a special 22 seconds after you killed the last one, then you’ll have gotten a total duration of 47 seconds (22+25) out of the pair of them, assuming you don’t kill something else to refresh the duration. If you kill a special 0.5 seconds after the last, you’ve gotten 25.5 seconds out of those two kills (assuming, again, that you don’t kill something else to refresh the duration).
Each special you come across is precious resource. As W40k itself put it once, “Life is the currency of The Emperor. Spend it well.” This is also why I recommend the revolver; its ability to one-tap things basically turns shotgunners and other enemies into a point-and-click refresh, on demand, unlike brainburst, which has to charge first, or assail, which can waste time writing your name in cursive for five seconds before they finally decide kill something.
I’d also add that I do not recommend the soulblaze method that people bring up. Now, I might have just gotten unlucky, but the 10% chance didn’t seem to proc very often. The In Fire Reborn perk gives you a chance of a warp charge when you “kill an enemy with soulblaze.” It’s possible it’s just misworded, but that implies that it only happens when someone dies to soulblaze specifically. While that seems fine on paper, one sweep of an Eviscerator can clear basically the entire swarm you just lit on fire, which makes that method very inconsistent in my experience, though others of course will beg to differ."