@EkadzatiThere is a daily delve that provides them as well; I forget the name, but it's the one where you must put out the braziers in front of portals (now and again, they are activated, resulting in one iron atronach as a 'last mob' through the gate).
Hope this helps!
The ones in the (daily) delve do *not* count against that achievement!
Hm, I'm not sure that's accurate, but will double-check. I have 60/100 and I promise you, I've not been that busy on the dragon cycle.
Just tested, they do not.
This is something I kind of discussed in another post. Honestly, wouldn't even mind the intense grind to get the 100 iron atronach kills IF the game REWARDED you with something worthwhile. Lol all you get is a few achieve points. I guess it's cool for the completionist but who has the time to grind all this stuff for basically no return? They really need to incorporate more substantial rewards for completing things. Maybe furnishing motifs, rare costumes, pets, mounts, etc. rather than just another variation of a gold dye or achievement points.