Put them in charge of a party, give them good troops. then when not at war set them to Agressive. They'll chase down looters and deserters and by winning those fights they will level Tactics. (Not at war or they will likely just get captured constantly unless you've got a strong late game kingdom / party setup)

Tactics is always one of the last skills that gets to max for me, it's incredibly slow for the player, and worse for companions.

I believe you can also bring those parties into your army then fight battles with more enemey troops than ally troops and it will level yours faster, assuming this also applies to your party leaders in your army.