I've killed more than 50 (a variety of enemy types and species) with the perfect 5th blast and my counter for the catalyst is still at 3/50. What gives? Yes I'm sure I killed them with the perfect fifth because they didn't die until it detonated, and no there wasn't any other players around killing them.
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Its not the explosion or the bullet, its the BURN that needs to kill them. Which is silly in my opinion. But yeah, it has to be the burn.