A couple of points about FM for this achievement:
a. Almost everything in the giant fort spawns relatively fast with exception of the two at the gate and the two in largest 'house' in giant fort are on like a 9 or ten minutes respawn
b. I am getting 100 kill every 11:30 - 12 minutes, not great (target a 100 kill every 10 minutes), though certainly managable.
c. Good for non-afk killing relative to Frostcrypt.
Frost crypt has a ton of giants, decent arrgo radius, normal respawn time 16 to 20 minutes. The issue is all giants summon in this zone making it difficult to get more than 4 - 8 mobs at a time. Painful for non afk killing of 4 to 8 mobs, better if getting 20+ per pull.