Go to the Castellum. There is infinite cabal spawn.

Friend_Within has it right. You need the burn to kill and NOT the shot itself. Meaning get your first 4 crit kills and then kill a full health baddie (eg Cabal) in the head and wait a second and the burn will kill him. Way and by far the best place to do this is in the Castleum. I have yet to run the Raid (just a matter of enough non-interrupted time not possible for >year now) but you can just run up there jump to the bottom and go back to the top of the stairs and just start shooting away. They will come non-stop and the only thing you have to watch for are the grenades so a strong Titan makes this extremely fast and easy to complete. Do a Youtube search on this, theres a few really good vid's (that's how I learned this method) Is it worth it. HELL YES! I run it all the time in strikes, NF, and PE's if you make the crit shot you get your bullet back and the perfect fifth gives the explosion so its, in my opinion it's a perfect weapon for any boss you are not going to 1 phase.

Hello, You need to defeat enemies with the burn DoT, not the actual explosion. For it to count towards the Perfect Fifth kills. I’ve seen other users complete that requirement in the Castellum of the raid.