I was disappointed too. I read The Giver and Gathering Blue as a kid then reread both and finished out the series as an adult.

I had moderate expectations for the ending, but I agree the "final boss battle" was kinda lame and I feel the intended audience (kids/preteens) would think so as well. I would've been more satisfied with a happily-ever-after, tied-up-in-a-bow ending instead.

I want to be lenient and chalk it up to: It's a children's/middle grade series, so it just didn't lend well to adult-me enjoying it.