Metal is very strong against them. The Warriors have a minimum armor save of 4+ (3+ with shields), so that's what you'll need to roll to wound them with Searing Doom. Their armor will be useless. Knights, if he has any, will go up in a puff of smoke. The Golden Hounds can also chip away at his characters if he fails his LOS rolls, but still puts wounds on the unit either way. And remember, any large unit deserves a blast from Final Transmutation.
Marauders aren't nearly as vulnerable, but then again your Saurus can get them. When you do get into combat, Plague of Rust and Transmutation of Lead will keep you ahead of the game.
Metal isn't as universally useful as Life or Light, but against the good armor saves of WoC, it's a good offensive selection.