Big boots to fill

To take over a job that someone else used perform in an impressive way. (‘Big shoes to fill” is also used.) 

“Our new teacher has big boots to fill, because our former teacher was so great!”

Tall order

Something that is difficult to achieve or fulfill is a tall order. 

”His doctor advised him to rest for a month but to him it’s a tall order considering his hectic business schedule.”

Two heads are better than one

When two people work together more things get accomplished. 

“I need your advice to improve our services. Two heads are better than one.”

Hit pay dirt

To hit pay dirt is to discover something of value. 

“The men hit pay dirt when they discovered oil in the farmer's field.”

Go ape

To go ape is to become very excited or enthusiastic. 

“I go ape over Christmas sales.”

Hold a grudge

To hold a grudge is to have a deep-seated feeling of resentment or rancor against another.   

“He holds a grudge against the manager for having reprimanded him in public.”

Hive of activity

It is a place where people have a lot of things to do.

“The cafeteria is a hive of activity at lunch time.”