I used the yellow bars on Mercury to do this, because there weren't too many other players around to ruin it. I timed it so I shot the enemy in the body with the fourth round and then plugged them with the fifth charged round and saw them burn. I screwed it up oh so many times. I hated how the catalyst requirents were done. Can't imagine how it can be done now. Definitely pick a place where there aren't many other guardians around.

Cabal are great for this. Proc the perfect fifth shot then target a full health red bar. Just shoot them “only once” in the head and the “burn” which is what the catalyst counts, will kill them after a second or two. Continued shooting will count as a bullet kill and it will not count.