Tactics, like your melee skill, only goes up when whatevere you're fighting has tactics in the same general range as your current tactics level (plus/min 20 I've heard). So what you probably want to do is, as GFY said, hunt orcs.
Or spend an hour in Haven shooting zombies and such, that may help too. When your tactics gets within 20 points of your archery, then you should gain normally on both with the usual methods.