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.