Formal:
* Intimidating
* Formidable
* Overwhelming
* Discouraging
* Demanding
* Herculean (implies extreme difficulty)
* Arduous (implies hard work and effort)
Informal:
* Scary
* Spooky
* Nerve-wracking
* Tough
* A tall order
* A real challenge
The best synonym will depend on the context of your writing.