Hand of Fate 2

As per the achievement description knock 20 enemies off ledges. I found this unlocked for me without even trying for it as the enemies will often get knocked backward when they die. This counts for the achievement as my 20th was a Captain who fell backward off the ledge when he died.

Look for combat maps that have bridges or open ledges and try to fight enemies while they are backed up to the edge of the map. Using weapon abilities seems to improve knockback on defeated enemies. I noticed this mostly with Empire and Northerners, less with thieves and probably impossible with Skeletons as they seem to disintegrate on death.

Hopefully this helps for people like me who weren't sure how or where to do this but fortunately it will probably come with natural progression of the game. Good luck!