Original Post — Direct link
I have seen several have problem with this, myself included, figured it out after a while.
You have to burn them the first time AFTER they re-animate. If you burn them when they are not animated you wont get the burn count.
So kill them "normally" first, then use fire when they get back up.

Clarification + TL;DR

Kill them with anything but fire, when they re-animate you burn them.

I used Firewall spec wpn for this.
over 4 years ago - UbiBlush - Direct link
Originally Posted by LucNukem73
ya but even if you burn them after it will not work... so dont use fire holster cause if he gets on fire before dying the first time, you wont get credit... Another great bug thanks to Massive... The last reanimated event didnt had this problem...
Hey, we have reported this issue to the team so they can look into it. Thank you for bringing it up.