"difficult endeavor" is a correct and usable phrase in written English. You can use it when referring to a difficult task or project. For example: "The journey ahead is going to be a difficult endeavor, but I am determined to succeed."
This continues to be a very difficult endeavor.
Developing RFID based applications is a painstakingly difficult endeavor.
Impeaching a federal official is an exceedingly difficult endeavor.
Assessing quality of life (QoL) in urban areas is considered a difficult endeavor.
In politics, sifting for the truth can be a difficult endeavor.
"This is a very difficult endeavor, but we are trying to show leadership".
In our conversation, I tried to get a sense of why understanding crime can be such a difficult endeavor.
Some success in this difficult endeavor has already been achieved — in the clinic, not just the laboratory.
His strength and expertise in this difficult endeavor will be greatly missed by all who labor in this important cause.
In such a difficult endeavor as the systematic development of biomaterials, computational chemistry is a natural partner for traditional chemists.
Finding an algorithmic criterion that matches the cartographers' judgment in ranking the significance of features on a terrain is a difficult endeavor.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK