That is debatable. There's a lot of games which have no challenge at all, especially creative ones like Minecraft's creative mode or Story-driven games
Viewing post in The Squire & The Jiggle Knight jam comments
Yes but that is contextual. Some story driven games have challenge in making decisions and dealing with the consequences. Minecraft creative is a playground to build in. Your game is an action/fighting game - the mechanic, the skill to test, is in giving the player the right directions. If it contained some more silly/emergent gameplay it could be more of a sandbox type experience but it isn't, you have a clear goal of dropping all the monster's healthbars to 0.