Metal: Hellsinger

EDIT (26/10/22): Guide’s gotten an overhaul as to the execution of this achievement, should allow you to get it in 10-15 minutes instead of 25.
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NOTE: According to Knitelight this achievement will also unlock if you take the cheese approach for

The Not Shaken, Nor Stirred achievement in Metal: Hellsinger worth 94 pointsNever hit off-beat in a Hell

If you want to cheese that achievement try that first and then see if you need to do this separately. Just make sure you either dash or shoot with Paz (and no other weapon) on-beat once in that run. As of 1/11/22 this note is redundant due to the added auto-rhythm feature. Just enable that before starting your Pazifist run and you’ll get both sorted at once easily.
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You are best off completing this in Voke due to the lack of non-Marionette enemy types (especially Stalkers and Elites) in this Hell. Make sure to also do this on Lamb (easy) difficulty to avoid fights going on for longer than needed and to have access to as many charges of Resurrect as possible.

As for the execution of this achievement, bring Ultimate Sovereignty with you if you can for dealing with Cambions and Behemoths; you don’t really need any other Sigil. For Marionettes, use Paz’s charged attack (which must be done on-beat if you’re not using auto-rhythm) to crystallise them, and then dash into them for an insta-kill (they will explode, but you won’t take damage). For Cambions and Behemoths, use Paz’s Ultimate to crystallise them (if the Ultimate isn’t charged when one’s around use the charge attack to quickly charge your Ultimate, should take 2-3 bursts depending on how many shots in said bursts hit a demon) and then dash into them for, again, an insta-kill. Unfortunately the Aspect can not be hit with the Ultimate or be crystallised period, so you’ll just have to spam the charge attack on her until she’s dead.

The achievement will pop about the same time as an achievement for finishing a Hell, in the unlocks/scoreboard screen.

This achievement can be very easily obtained! Go in to settings, under accessibility at the very bottom there's an option to "always hit on beat." This means you can just fire away and every shot is a "good" shot. Will void from leaderboards but still unlocks achievements. I recommend doing this on the second level as there's only one weapon pickup (if you're doing this on first play through, when you pick up the weapon immediately switch back WITHOUT FIRING to the Paz.) The Paz is the skull weapon make sure you use it throughout the level. You can still execute/slaughter and use your ult ability but you can not use any other weapon. Dashing into enemies is also ok. Doing this method can also help with other achievements such as beating a Hell with on beat hits only and hit streaks/hit streak kills.