This is the best game I played in years and years. And I'm not playing another console RTS unless it's as good as Brutal Legend.
The problem is that Double Fine always meant for it to be a multiplayer game. EA had the brilliant idea of marketing it as a single player game.
That shouldn't suggest it was a secret. There are PLENTY of multiplayer videos. But if you want to get the most out of Brutal Legend, multiplayer is where it's at.
If you are having trouble with that part, make sure you collected all the guitar solos, and when the tower goes down, cast Bring it On Home, Light of Dawn, and everything you possibly can.
See the thread on my multiplayer guide for help as well.