difficult undertaking; uphill task
Refers to a project, business, or task that is inherently challenging and requires significant effort.
This plan is truly a difficult undertaking.
Developing the new product was an uphill task, but the team overcame it.
大事業 is a large-scale project, not necessarily difficult; 難事業 explicitly highlights the difficulty.
Compound of 難 (difficult) and 事業 (undertaking, enterprise).