challenging

When all this was done, and the army disciplined, and the herald Mouse had duly proclaimed war by challenging the Weasels, the newly chosen generals bound their heads with straws, that they might be more conspicuous to all their troops.

If he were afterwards to marry Mademoiselle Danglars, you would accuse me of intrigue, and would be challenging me,—besides, I may not be there myself.”

Instead of fifty repetitive math problems, students might solve five highly challenging ones.

When we confront a challenging project, we are frequently struck by a wave of uncomfortable emotions, such as anxiety, boredom, or profound self-doubt.