WoWDB

As of Patch 5.1, this is one of the two rarest Battle Pets (along with the  Scourged Whelpling).

Best method I have undertaken to get this pet is to kill all the tainted rats in the area (shale) then go outside of shale, and kill all the battle pets (within minutes of killing the first rat) that you can find. around the outside of shale. Then go back  and wait for the "tainted" rats to respawn, they will either be rats again, or a chance at the minifernal.

This method forces the server to roll at a chance of adding battle pets to the shale, as battle pets had recently been "defeated" and the killing of the rats opens up slots for battle pets

The caption specifically says, "...sometimes rise from the remains of larger infernals who fall in battle." Has anyone tried killing the larger elementals and see if one spawns?

I parked an alt in the spawn area on a medium pop server and logged in about 10 minutes before the server shut down for maintenance. There was no one else in the area, and I found one in the north, hiding behind a tree.
My other realms are high pop, and there's always at least 6 people circling around the place waiting for spawns. So realistically there's no point in camping them if that's how it is.

I'd suggest maybe making a death knight on a low/medium pop server and just leaving it in Felwood to log in time to time. Maybe if you stay up late, or have an early morning shift, take a few minutes to sign in and check the area.

Camped for 29 hours of /played and yet to even see on spawn.  CRZ makes this one of the most difficult pets to capture.