Cinderfoot stags can be found in northwest Celestion, in Emberwood. *Note* They actually spawn randomly in Emberwood, and despawn as soon as they run out. Here's what I did:  I found a spot (check screenshots) where two groups of stags would spawn and run, one on the left side and one on the right side, spawning alternatively. Just run into the cinderfoot stag, and he'll start casting a red telegraph. Stand in it and you should receive the achievement.  As of 09/13/14, the achievement still appears to be bugged. I got trampled 12 times before giving up. No credit.

Cinderfoot Stag spawend random in Emberwood in a group of three, two Cinderfoot Runner und one Cinderfoot Stag

Still doesn't work! Have been run over numerous times and the lowbies are giving me weird looks. 1/18/15

Got me on his first attack. My friend tried repeatedly and several spawns and still hasn't been able to get it to complete. Not sure whats going on with this one.

I left my character 10 min there, and the cinderfoots hit me like 12 times.... no achievement :/ still bugged

It took me around 10 times to get the achievement. At 23/06/2014 it doesn't register the knockdown all the time.