The phrase "difficult work" refers to a task or job that is challenging, requiring a lot of effort, skill, or persistence to accomplish successfully. Full definition
It empowers them to figure out even difficult problems by doing the complex and often difficult work of thinking through the problem themselves. (edutopia.org)
But if you're looking to do difficult work and truly need a heavy duty truck, you really don't have to look much further than this truck. (bestride.com)
People often start off their day by completing easy tasks to get themselves rolling and leave their more difficult work for later. (businessinsider.com)