Overwatch 2

This, just like many other achievements, does not require you to actually get 4 kills. You can get like 2 kills and have been assisted on the other two, as long as they die while in your ult.

What i did was play defense on the payload and let the enemies clump up near the objective. I used Reapers teleport ability to sneak behind them, and just used my ult while my team was also damaging them. Essentially, it relies on timing and luck.

Reaper's Death Blossom ultimate ability allows him to spin a circle shooting off all the rounds in his shotgun dealing damage to all enemies within range.

We will need to kill 4 enemies in 1 activation of the ability. Also long as the elimination comes up on screen, it will count, you don't need to deal the final blow or get the majority of the damage. Use your movement abilities to get behind the enemy team, get up close and attempt the achievement.

I recommend going for this while defending the payload and having multiple enemies on the objective at the same time. It can also be done in any "capture" point that has many enemies, both in attack/defense.

If you can get a total of 4 kills at once, you will unlock the achievement.

If you'd like to learn more about Reaper, you can view this video. The achievement is described and shown at 12:36:

What Dominic890x said! It's very important to remember that assists count, as long as you're still in your special.

Here’s what I did:

1) I unlocked this on Watchpoint: Gibraltar in defender Escort mode (as we do with most Achievements). The payload is in a nice, contained area at the beginning and end of the fight; it seems to take forever.
2) Charge up your Death Blossom special through normal play.
3) I waited until right near the end of the match, when players will group together at the objective in a final push. Bit Y on the Xbox One controller when that happens and cross your fingers. You’ll need to have four people die at your hand — or, equally as good, as an assist — while you’re still using your special.

Voila! Here’s how it all looks, keeping in mind we made the video using XboxDVR.com, so the Achievement doesn’t actually pop on-screen.

I got this achievement in an unusual way. If you get interrupted while using Death Blossom and still have a portion of your charge, you can use it again once it's charged and get the achievement. The video provided shows me getting interrupted shortly after a kill by Roadhog and then later on, using it again and getting three kills and the achievement popping.

This achievement took me about 6 hours of game time to unlock [!], so I thought I'd share how I finally managed to get it for others who might be stuck.

Like the other (great) solutions here, mine relies on a bit of luck. My team was on offense on Hollywood, and the defending team managed to stop our payload right on a corner in the western-themed section of the map. I snuck around behind and above the enemy team through one of the buildings, and happened to get my ult just as their team had bunched together on the payload and my team's Roadhog AND Hanzo launched their ults. Between the three of us, we wiped out five of the enemy in about 1 second. Achievement Unlocked

Hope this video helps! At the very least, it'll give you another idea of the kind of positioning you'll want to look for to both stay alive AND achieve maximum carnage.

Reapers ult known as death blossom can devastate a group of enimies as you spin around shooting your hellfire shotguns at everything around you.

for this achievement, we're going to need to get 4 kills with one use of this mighty ability, while it may not seem hard, youve got to realise that, unless the enemy team WANT to die, theyre not going to hang around when they see a reaper and hear the words "DIE! DIE! DIE!". theyre going to run for it.

heros can stop you by:

genji: deflecting at the reaper kills it every time

mccree: stunning stops it immediately

mei: her ult freezes everyone in the area

zarya: can pull you away from the objective towards someone who can kill you with her ult

junkrat: his mine can rocket you through the air onto a ledge higher up, meaning you do no harm to anyone

tracer: recalling behind cover until youre done and go in for the kill

bastion: bastion can deal more damage than you and can quickly focus and kill you before you can kill him

sombra: if she hacks you, from above or with emp, it cancels any ult ability

widowmaker: snipe you rom afar

ana: pretty much the same but she can also put you to sleep which, like stunning, stops it immediately

roadhog (not likely to try but still might): roadhog can hook you in but as soon as he does, even though your ult is over, you can wraith to ready up then go in for the kill

(evey hero can if you dont catch them unexpectedly as they can work as a team to quickly focus and kill you)

the best maps for this are:

dorado: (see bottom bit)

kings row: a lot of ledges to drop down from as long as the junkrat doesnt see you

worst places are places where players can easily hide behind something

ok so i got this is the dorado payload whilst attacking
as we came through the gate
into the sort of festival bit, i dropped my ult, devastating
4 enimies at once and getting us through the payload, i hid
behind the payload and just went for it when i saw that my team
were all dead and, instead of panicking, i just let out my death blossom
and wiped the whole field around it.

hope this helps :) sorry for any spleling mistakes, i type fast and dont look at it lol.