Churning Earth

Do you play in HARD MODE?
Are you an Elementalist?

You need to check out this skill
Churning Earth

Create Churning Earth at target foe’s location. For the next 5 seconds, Churning Earth strikes foes near that location for 10…34…40 earth damage each second. Any foe moving faster than normal when struck by Churning Earth is knocked down.

This skill is very effective in Hard Mode, since the enemy’s increased movement speed will result in a knockdown every time they move, even if they aren’t moving faster than they normally should be.

“normal speed” actually means the speed you as character move with normally.
this being said everything that moves faster than you will be KD.
Hard Mode creatures move 33-50% faster than normal (wiki)
so they will be KD.
even when slowed for lets say 25% they still move 8-25% faster than you and will still be KDed.