So how would your anarchist utopia deal with an army of thousands raiding your village?

> the rest of your post rather depends on that point of view.

The need to feel safe from raiders is fairly universal.

"Thousands of raiders" don't magically appear, you prevent them by preventing the conditions necessary for thousands of people to unite with the goal of hurting others.

How would you prevent that without a power structure?

How does a power structure prevent it? Raiders are a result of people unable to live off their of own labor, generally as a result of the exploitation of others.