# Behold

This can be translated as "Look" or "Listen" or "Pay attention to what I am about to tell you."

# will lop off the boughs

He will destroy the army of Assyria like strong men cutting off the big branches of trees. 

# with a terrifying crash

"and the branches will crash down on the ground and make a terrifying noise" or "and the branches will fall to the ground with a very loud noise" 

# the tallest trees will be cut down

"he will cut down the strongest soldiers" 

# lofty

proud