Generally when I want to wipe an enemy that I can't do without heavy losses, I park my army in a base or two somewhere then temporarily load up on a "trash" army. Basically full of low level units that I would never use for my actual army. I then destroy the opponent without caring about losses. I basically view it as throwing money at the problem.
That's how I do most of the castle sieges and other big fights in my game earlier than I would otherwise be strong enough to without losing a lot of guys.