# a sharp threshing sledge

A threshing sledge was a board with sharp spikes that someone would drag over the wheat to separate the grain from the chaff.

# two-edged

This refers to the edges of the spikes that were attached to the threshing sledge. That they are "two-edged" means that they are very sharp.

# you will thresh the mountains and crush them

"you will thresh your enemies and crush them as if they were grain, even though they appear to be as strong as mountains" 

# you will make the hills like chaff

The hills are a metaphor for the powerful enemy nations near Israel. People having the wind blow the chaff away after they have threshed the grain means that they allow Yahweh to destroy their enemies.