Training videos

You might also check some of the Sound on Sound articles. They'll augment the Groove3 stuff and perhaps give you some insight into any specific techniques for metal although from what I've read there and in Mike Senior's book, "Mixing Secrets for the Small Studio," the main difference between genres isn't so much technique as it is reference tracks.
 
Find a few tracks by artists in your genre, tracks you think are similar to what you're trying for in a given track you're working on, and keep them handy for back-and-forth comparisons to (forgive the pun) keep you on track.