"Working hard" is a common way to express that someone is working very hard and putting in a lot of effort. Other phrases that you could use to convey the same idea include:
Putting in a lot of effort
Putting in a lot of overtime
Burning the midnight oil
Working long hours
Grinding it out
For example:
"I've been working hard all week to meet this deadpne."
"She's been putting in a lot of overtime to get the project finished on time."
"He's been burning the midnight oil trying to finish his thesis."
"We've been working long hours to get everything done before the deadpne."
"They've been grinding it out to make sure everything is perfect for the event."