Metal: Hellsinger

Level: Slaughter Mastery III

Strategy: Build Paz’s Ultimate on first wave of enemies, then stagger/weaken 2 of the 3 Seraphs with 4 on-beat shots from Hounds. Use Paz’s Ultimate which temporarily makes nearby enemies frozen and combustible. Shoot frozen Seraph, blowing up other Seraph in the process (if it’s close enough). Achievement unlocked.

During all this you will likely need a slaughter or two to keep the timer fed. Marionettes take 3 on-beat Paz shots to stagger, and Cambions take 2 on-beat Hounds rounds to stagger.

I post this because grinding an entire level for one chance wasn’t ideal, and 3 Hounds Ultimates that each killed 2+ Seraphs didn’t pop the achievement.

Level: Nihil
Recommended Difficulty: Lamb

Recommended Weapon: Persephone (Shotgun) and Hellcrow (ranged blades)
Recommend Sigil(s): Last Breath Aegis (Double damage on low health) and Ultimate Sovereignty (Auto charging ultimate)

Two Seraphs will appear in the final room before the boss in this level. They are the flying enemies with the large blue wings that can shoot laser beams and explode in a bubble.

Clear the final room of every enemy until only 2 seraphs remain. Hit each of them with 1 attack of the Hellcrow to weaken them. Get hit to be on low health, and you'll notice the music gets an effect added to it to signify your weakness. Now, take out you shotgun, and line up both seraphs to kill them both with the ultimate shot. You'll have to be somewhat close for it to connect, and it should do A LOT of damage if low on health and shot on beat; instantly killing both and unlocking the achievement. If you accidentally kill 1, you'll need to restart the level. If you miss both, or fire off beat and they survive, wait for the ultimate to recharge to try again.

A much easier solution for this achievement is to play "Slaughter Mastery 3."
This will save you having to replay an entire level to just have a chance to get the achievement. 3 Seraphs spawn after you kill the first wave. Get them down to a staggered stage with the "Hounds" and then shoot them 1 more time, then activate your ultimate and the ultimate should kill 2 at the same time, or very close to each other. Mine popped when the ultimate killed one after the other but the gap between each kill was almost instant.
It will also help to kill any other enemies before you activate so its just the Seraphs around so your ultimate doesn't try shooting something else. When the Seraphs spawn, only 1 or 2 small enemies should spawn with them.
This may take time if you are unlucky as the ultimate can obviously decide not to shoot the Seraphs, but its easier then playing an entire level to only have 1 chance to get it.
I personally got it on my second go trying this method.

Video method for doing it on torment challenge slaughter mastery 3

Timestamp: 4:07

Easiest for me (after failing at Slaughter 3 for almost an hour)
Hell: Nihil
Difficulty: Beast
Weapons: Persephone (Shotgun), Hounds (Revolvers)
Sigils: Ultimate Sovereignty

In the very first arena, two Seraphs will spawn, but only if your difficulty is Beast or above. When on Lamb or Goat, only one will spawn.

Fill up Paz's and Persephone's ultimate on the first two waves. After Seraphs spawn, try to get them close to you. Don't shoot them with Hounds or you risk one of them dying too early.
Pop Paz's ultimate, that will stagger them. You do need to be very close to you and each other, so double jump right below them. Now you can either shoot them (if they are close enough) with any gun, causing both of them to explod, or use Persephone's ultimate to get both in one shot.

This took me whole two tries. I would add a recording but Shadowplay doesn't like the Gamepass version.