Snuff the Flare Reapers  

Assassins, gladiators, impalers and reapers all roam in the same place, I am not sure the rhyme or reason for any order they spawn in each others place, but don't expect reapers to spawn if your only killing reapers. I have had as many as thirteen up at onces and as little as none.

This quest will be horribly lengthy unless you get a full group.

Reapers are found in the area with lava pits before the temple of ro (spire 5).

At level 70 i soloed the area, clearing PHs and i got about 10 reapers to spawn each hour. We are talking 5 hours in total for the quest, soloing at 70.

There are four different kinds of flame fiends in the area: gladiators, impalers, assassins and of course reapers. To find out which shares spawn i did a simple but boring experiment. I killed of everything that wasnt an impaler and checked the tracking window for changes. Eventually everything had respawned as impalers which proves that all four kinds share spawns.

Your best bet is to just run around and kill all flares you can find and the spawn chance for reapers seem to be around 1/4. The respawn time is somewhere between 10 and 20 minutes but hard to tell since they move around a lot and you can not easily link one kill with a certain respawn.

best of luck