re: Need help remembering a book series and I know the TD sleuths can come through.

I think your memory is combining two different trilogies into one. I'm not familiar with The Iron Tower Trilogy, but the second part of your description sounds like The Wishsong of Shannara, which is the third book in the original Shannara Trilogy by Terry Brooks. There is a character named Garet Jax, who is a bad mfer known as "the weapons master." At the end of the book he duels a huge red apelike creature called a jachyra. I read the series as a kid and listened to the audio books fairly recently as an adult. It holds up well in my opinion.