Raining Blood achievement not unlocking

[Tufwfo]

I went into Sheol and got to the part where 2 seraphs spawn next to each other and killed them both with the shotgun's ultimate. It's my last achievement so it would great if this could be fixed unless I am somehow doing it wrong

[Blockhead]

I managed to get it on Acheron - there's a section where two seraphs spawn twice in the same area. I used Vulcan, and made sure both of them, individually, got down to low enough health that it allowed me to slaughter them. Once they were both low enough, I used Vulcan's ultimate to pull them together in a corner, then used Vulcan's primary attack to blow them both up at once and got the achievement.